It depends very much on a situation.
If some characters are falling to a chasm then I don't think the size has any effect on damage.
However, falling from a large tree is different for humans and haflings, for example. For me at least. For the record, halflings are small
Consider a situation when a grown man is falling from the tree and a child. My money is on the child.
Spells complicates things even more.
Tossing a fireball at a dragon you don't usually consider it's size. I mean, white dragon can be quite big
Gargantuan means 20x20 ft. that is 4x4 tiles on standard tactical map.
So, theoretically, fireball can reach all 4 front (let's call them that) tiles of a dragon. Aiming at the middle of the dragon you'll get a half sphere with a 20ft radius, so 40ft of diameter over the width of dragon and 20ft perpendiculary to this.
In my opinion if a ice storm has damage per tile effect then fireball also should. How about fire storm? In my games you can deal 70d6 dmg with fire storm(7d6 per cube). To provide some context, Meteor Swarm deals 40d6 dmg.
On the other hand, meteor swarm's description explicitly says: A creature in the area of more than one fiery burst is affected only once.
I'll say talk with your gm and decide how you'll be calculating this. I did't find any rules about this type of AoE damage.
In my opinion "damage per tile" effect will be overpowered, I don't think D&D was balanced with this idea.