I do understand that randomness can be frustrating to deal with, especially when they lead to death or other unfavorable things. Recently in a fight, I had a chaotic witch with no damaging abilities on her own attempt to cast a spell upon the Player in question, Constitution save, DC 10 to either be enlarged or reduced, they succeeded, enlarged and proceeded to kill off many of the witch's minions then the witch herself, breaking the concentration of the spell the witch had, returning the Player to normal size. Yet a few days later they told me they don't like random effects, have never played a D&D game with random effects and does not want them in the game. I'm here scratching my head in a bit of confusion over this as the rest of the party has been having fun, some of them jokingly said they wished they had been hit with enlarge/reduce effect. Any advice on how to handle this? Do I just refuse to target them with spells/effects that ask for saving throws or just tell them that it's part of the game, or whatever other suggestion people may have.

If they don’t like randomness, why do they play a game with dice?!?

Just an FYI: Enlarge/Reduce is not random, the caster chooses which effect happens.

I don't understand this either. The entire game of D&D is about using a random number generator (dice) and then RPing about the result. If they don't like random effects, they should play a diceless game or a consent-MUSH.

I would have a talk with this player out of game and explain that random effects can and will happen. If they can't handle it, you will need to explain that they should leave the table. Or else talk to the whole group and agree to play a diceless game or something.

I don't think the player dislikes actual random effects. I think the player dislikes having things happen to the character that couldn't be predicted. To which I say the player is going to have to suck it up or stop playing. It is not possible to play even a single session of D&D without both random and unpredictable things occurring to the characters.

Quote from RegalMasquerade >>

 Recently in a fight, I had a chaotic witch with no damaging abilities on her own attempt to cast a spell upon the Player in question, Constitution save, DC 10 to either be enlarged or reduced, they succeeded, enlarged and proceeded to kill off many of the witch's minions then the witch herself, breaking the concentration of the spell the witch had, returning the Player to normal size. Yet a few days later they told me they don't like random effects,

 I think they mean that they want their fantasy to be realistic.  Coming across a witch who has a dubious list of prepared spells that she casts from randomly and then randomly choose some side effect that turned the fight against her is perhaps just too silly and non nonsensical for the games they like.  Next time make the boss smart and force them to fight strategically.  I'm sure they don't mind something random if it clearly makes sense to the plot and to the world.

I have a number of questions, the first and least important of them being: Why the heck would a witch cast and maintain concentration on a spell that actively harms her interests?!

More pertinent (and important) however, is what do they mean by random effects?  Because there is a difference between being subject to the randomness of dice and the randomness of things like a Wild Magic table.  If it's dice rolls that they're objecting to, then yeah, D&D probably isn't the game for them.  I think it's more likely though that what they're objecting to is being subjected to something like the Wild Magic table when they aren't playing a Wild Mage or anything equivalent.

Have another talk with your player and see about getting some more clarification on what it is they're objecting to.  If they are objecting to randomness like Wild Magic, I think that warrants another discussion about what the table wants and is or is not able to compromise on; It may also be worth considering that this might not be the table for this player.

I suspect it's the effect of "too much randomness" rather than the fact of the randomness existed. 

Both too little and too much randomness can be unfun. If nothing is random and everything can be mathed out, then the game starts feeling like an unfun optimization puzzle. If nothing is random because the DM just fiats everything, the game can be unfun because it becomes all about persuading the DM (though diceless RPGs can definitely be a thing, D&D isn't made to be one.) 

If you have just the right amount of randomness, the game is pretty fun! You have enough control over the situation that you can make plans and execute them, but enough randomness that you can't just optimize all the way through to the end and have to react to unexpected things, there's luck and suspense, it's great.

But if you have too much randomness, it starts to feel that none of the choices people make matter. You prepare for this battle, work on your character... then the battle starts and some character gets enlarged and squishes everything. Maybe next battle they'll try to plan but then just die to some other random effect. What's the point of planning and solving problems when everything's so random you might as well just walk in, roll the dice, and see what happens? 

That particular player probably felt like, that battle, no choices they made mattered. If they failed the save, then I guess they're reduced, squished, and out of the fight. If they succeeded on the save, they're enlarged, and all of their previous character choices *also* stop mattering because they just crush everything. Oops. 

My guess is that the part of this whole thing that made the player feel bad was that they passed the saving throw and were still subjected to what they thought was a really frustrating effect. Generally, the game maintains the pattern that if you fail the save, you get some harmful effect, but if you pass the save, the harmful effect is either halved or removed entirely. Whereas you made a spell that had an effect no matter what they rolled, it's just the kind of effect was selected randomly. 

I think there's a pretty big gap between "making a saving throw to avoid damage" and "drawing from a deck of many things", and the player felt like you were steering much closer to the latter. 

I also dislike situations where "random" monsters attack my party that have no apparent reason to be in this area in the first place.  I like the "random encounters" to fit into the area, region and story.  So I don't have a problem encountering more bugbears, orcs, or goblins in an area where we have been fighting similar themed monsters, but when I am in the mountains, I would dislike my DM rolling, consulting a table, rolling again, and announcing that the party has been surprised by a dozen giant frogs, when nothing about the situation ever suggested frogs and we are up in the mountains for goodness sakes.  I don't mind the random chance of having an encounter, but I don't like meeting something that doesn't fit in with the environment or story at all.

Likewise, while it is not enjoyable, I dislike it when I have a skill challenge and my modifier is quite better than my opponents and I fail to win the challenge, but that is just the breaks.  In the moment, it feels like, "How can I miss this challenge when I have a +10 modifier and they have a +4?"  But this really means I have about a thirty percent chance of winning over my opponent, so pull up your big-boy pants and deal with it.  Losing such a skill contest feels bad in the moment but it happens.  Remember, once in a while you win one you should have lost.

I know it isn't random but I had flavored it to be so with the Witch I was using since the main theme of the setting is the land the party had found themselves upon has been magically twisted altered and the party is trying to learn more about what is going on in order to try and fix it

Sounds like it isnt randomness they don't like but rather whackyness.

It sounds like the situation was along the lines of the witch cast a spell but rather than harming the party it turned out helping them kill her minions because the magic was uncontrolled.

Maybe they don't like enemy tactics that don't make sense, as the witch would have dropped the spell when she saw it turning south. 

Quote from GaspodeWD >>

Sounds like it isnt randomness they don't like but rather whackyness.

It sounds like the situation was along the lines of the witch cast a spell but rather than harming the party it turned out helping them kill her minions because the magic was uncontrolled.

Maybe they don't like enemy tactics that don't make sense, as the witch would have dropped the spell when she saw it turning south. 

 Yeah, that’s part of the reason I ban Wild Magic as a PC thing in my world. Randomly growing taller or older or blue is a big turn off for me.

Quote from RegalMasquerade >>

I know it isn't random but I had flavored it to be so with the Witch I was using since the main theme of the setting is the land the party had found themselves upon has been magically twisted altered and the party is trying to learn more about what is going on in order to try and fix it

 So what they player thought was "random" can either be the twisted magic working differently, or some other logical reason.