Coming at this from a pre-3e perspective.
Ok, so assuming Fireball is a 3rd level spell, the mage gets it at 5th level.
Assuming Fireball is still a d6/level damage spell, that gets you 5d6.
Assuming maximum damage = 30hp. Saves/reflex for 1/2 = 15hp dmg.
Assuming the area of effect is 10' per level, so a 50' diameter flashbangburn.
Kobolds: 2-4 hps...maybe 5 or 7 for a chietain or something. Even saved, they are blackened burnt crispy critters.
Goblins: 4-6 hps. Even saved, they are blackened burnt crispy critters. (taste more like chicken than the distinctly cajun flavored kobolds.)
Hobgoblins: 6-9 or 10 hps. Even saved, they are blackened burnt crispy critter (taste more like turkey than chicken...but the charcoal aftertaste is nasty!)
Orcs: 8-10 hps, let's say maybe up to 12 or even 15. Even saved,...you seeing a pattern? (Naturally, they taste like pork ribs minus the BBQ sauce left on the grill for about an hour too long.)
Bugbears:15-18 or 20 hps. Saved are going to be scorched and blackened, but still standing, barely. Those that don't save,...hmm, what would blackened burnt crispy bugbear taste like?
Works for me. I have no problem with 5e using this as a benchmark.
Naturally, since most chamber we find ourselves in are not more than 50' across, there's bound to be some scorching and charring backdraft.
--SD