<Snipped quote by rush99999>
I thought the movement section covered it, but I was wrong. I've looked in Monster size section to see how big Huge monsters were in comparison to Medium sized PC's, and it's not there either. -_-"
This is the best I could find to explain my reasoning.
Basically, I thought that if a creature took up 15x15ft on the battle map, then their 5ft movement would affected by the fact that they're a size large.
They move the same as any other creature, if you wanted me to chime in with my experience as a DM. Even if a Giant is 100 feet tall with a move speed of 30
Feet, they move 30 feet (I’m exaggerating an example, most huge or gargantuan creatures have quite a lot of move speed to represent huge strides, etc, but also unless the snake has a way to climb really well (which they probably do!) climbing takes +1 foot of movement per foot moved as well! So many weird movement rules ).
But because 30 feet = 30 feet = 30 feet for any creature movement regardless of size, ray of frost is a nasty little cantrip.
Size is purely how many squares a creature inhabits, carry weight, and grapple potential; we can’t grapple THEM (two sizes larger than us!)… but they sure as heck can wrap us up!
Don’t be disheartened or take any mistakes negatively; 5e tries to be as simple as possible, so unless a sheet or specific rule says otherwise, things all follow the same simple core mechanics. Mistakes happen and, honestly, the game isn’t meant to be taken super hardcore as ‘written in stone’ so they’re usually benign and fine to have in a game. If we desperately need something to be ‘absolutely correct’ that’s what ooc discussion is for!