Figure out the highest level spell a spell-caster would need -- any of the spell-casters. Then, everyone's that level.
Unless, of course, you have to hold back the wizard to prevent his having spells at the level you need for the cleric. . . just not letting him have some spells could work, provided you don't need him to, say, cast a fireball at that level, so he can do THIS much damage, but not THAT much damage. (Having it do low damage by dice rolls is blatantly the hand of the author shoving things around.)
Plus, of course, some wiggle room to let them rise in level if the story requires that. Structuring it to cope with more powers as they rise in level can be full. To be sure, you want to avoid giving precise stats and/or mechanics so there is still more wiggle room. But "max spell level" works pretty well as a rule.