Xeviat
Dungeon Mistress, she/her
Assuming the adventuring day that the game is balanced around, a 5th level wizard can cast three Fireballs a day.
Fortuitously, that is the same number of Action Surges that the fighter will get, allowing us to balance them against each other.
Unless you're making concomitant changes in casters, changing the capabilities of non-casters is an ideal time to adjust the power of the change to bring them in line with casters - wherever you happen to think that line is.
It would be interesting to take a deeper look while I'm in there. I think the Wizard should be a bit more offensively powerful than fighters and even rogues, because the wizard's defenses are so much lower (lower hp, lower AC). But, it's probably not too many.
Also, fireball and many spells deal more damage than the spell creation guidelines say; then again, I'm sure the no damaging status effect spells are better in the long run than most damage spells. 8d6 fireball is 28 average damage, though, and 4d6*+4+weapon specialization is 41.33 with action surge, so it has it's benefits. Beefing up Maneuver options could be a good place to shore up combat disparities, though.