We've hit a peak in our campaign where we're essentially being assisted by ourselves from the future when we are at level 20. We will be playing those versions of ourselves, and are being allowed to design an artifact weapon to fit our characters. I have a great idea in mind for my Lightfoot Halfling ranger/rogue, but the thing is, the damage bonuses are reliant on the size differences between my character and the target. In short, for every size my target is larger than myself, I gain a +1 to hit and an extra damage dice (longbow, so 1d8). There is also a 'favored enemy' variant where the weapon gains +3 against a chosen creature type that is chosen at dawn, and a +1 against all others.

For this particular battle, we will be fighting off hoards of zombies and a gargantuan monstrosity with potentially large or huge sized champions thrown into the mix, and actual creature types will likely vary as well. Is it possible to have the size variations and creature types of my potential targets a toggle or selector before I roll to my VTT?

There's no way to do conditional modifiers like this on D&D Beyond.

I think your best option would be to add multiple copies of the weapon to your sheet, and then customise each one with the appropriate bonus for scenarios you expect to come up. Personally I'd just do the variants for size, e.g- "Bigger They Are Blade", "Bigger They Are Blade, +1", "Bigger They Are Blade, +2" then just make sure you're rolling the correct one.

I don't use any VTT's like Roll20, but for Avrae you might want the base version to be "Bigger They Are Blade, +0" so you can do a command like "!attack +0" which should then match the correct weapon without having to disambiguate it, assuming you don't have any other +1 or whatever weapons.

The Favored Enemy thing is more complicated; if by creature type you mean aberration, monstrosity etc. then you have no idea what size they're going to be when it applies, so it's probably best just to add that manually when it comes up.

For Avrae you could also potentially use a single weapon and snippets like so:

!snippet plus1 -b +1 -d 1
!snippet plus2 -b +2 -d 2
!snippet plus3 -b +3 -d 3

So to attack a medium target you would do:

!attack Bigger plus1

If it's your favored enemy you'd do:

!attack Bigger plus1 plus3

Not pretty but it's about as easy as you're likely to make something like this.

Okay, that's what I was afraid of! But thank you very much, I'll go into it with this in mind. I think it will be best to just make multiple copies and toggle the appropriate one as attuned during the battle to make sure the damage rolls are correct.