caribet had a nice response to such a thing in an earlier thread:
https://forum.atlas-games.com/t/targets-size-and-mass/990/1
So, for a really DnD like experience :
Inferno of the Mage's Ire
AKA The Devil's Sphere
Level 50
R: Sight, D: Mom, T: Ind
A pellet of flame shoots from your hand to the indicated target, and then explodes into a massive fireball, 30 feet across, doing +25 damage, reducing everying in its way to ash.
(Base: 20, +3 Sight, +3 Size)