I'm curious, just how tall are Stone Golems supposed to range at?

Depends who builds them but large creature so anywhere from 10-20' i guess

What is supposed to happen when a Golem dies? Do they just keel over like most creatures or do they crumble into dust? I suppose its probably up to the DM but I like to be consistent.

Probably remains as a shell
The animating magic might collapse causing it's body to fragment (as clay golems would likely turn to literal clay sludge due to being clay)
But I think it's really up to the DM. 

I'd run it as the stone golem falling apart, with most of it's body fragmenting from the lack of magic holding it together.
But that's just me

Uh, this thing has Magic Resistance but not a single saving throw in which it's proficient?!  Hello House Rules ...

Why would you need to houserule it

To match a personal opinion of how challenging I believe a challenge rating should be.

i dont know?

It can cast Slow. At will. It’s only slightly less awful too! Terrifying!

Advantage is mathematicaly equal to a +5 to all saves vs magic. No need for extra nonsense.

Yeah, that may be, I'm not sure, plus it has all those immunities.  Look at my thinking as a much older editions mind set coming at the new edition and not being able to make things match with the internal pov.  It's an in process sorta thing.  :/

How big and tall are stone golems? I'm trying to make a miniature of one or more.

Keep in mind that DCs tend to be not as high as previous editions.

A level 10 player character (appropriate to its CR) would have save DC of 12 + ability bonus. It's still unlikely to make Int or Cha saves, but the rest should have a fairly solid chance with advantage.

 Notice that its Slow action works just like the 3rd-level Slow spell, with the exception of the rolling d20 for all spells cast and the -2 penalty to AC and Dexterity saving throws. Since it's extremely close but the spell is slightly more powerful, if I wrote the stone golem, I would simply give it an Innate Spellcasting trait allowing it to cast slow at will. 

In Candlekeep Mysteries they have a Stone Golem. They describe it as 14 feet tall and have an interesting mechanic for death and dying. I think this could be applied to other Stone Golems.

Over the course of the journey, the golem might face mortal peril. If it drops to 0 hit points, the golem stabilizes and topples over but isn’t destroyed. It doesn’t make death saving throws, and it can be revived and repaired using healing magic. If it has 0 hit points after being struck by a disintegrate spell or similar magic, the golem is obliterated,

but the sapphire is left behind.

 

Quote from Spl3rge >>

What is supposed to happen when a Golem dies? Do they just keel over like most creatures or do they crumble into dust? I suppose its probably up to the DM but I like to be consistent.

 I'd say they would fall apart. The only way a stone statue could survive 178 HP worth of damage is because it's a golem. Basically magic that animates it is the only thing holding together, once that's gone the statue crumbles from the massive beating it has taken.

Why is this creature not immune to stun?

not sure, cant imagine this thing on the ground sputtering in shock

Can this thing heal? I thought that it was something like 'once a rock breaks off, it stays off.'