Would a pokemon's formes go together with a mega evolution? I doubt it, though as far as I know, there's nothing that says it doesn't.
But just in case it does, I figured out how much bigger that would make a theoretical maximum size pokemon.
With that, the maximum possible size increase from regular formes, mega evolutions, alolan forms, and totems, would be ~17.71x
How I figured this out: https://ideone.com/ijICc9
Oh, and fun fact: when you add fun things like Alolan Exeggutor and Hoopa Unbound and Wishiwashi School
, that number goes up to ~1043x, which is so big that pretty much everyone will immediately call bullshit.
I wasn't really content with just having a mere 5.67x multiplier anyway.