In Dungeon World, human clerics and wizards each get to pick a spell from the other caster class's spell list. Must the player choose a level 1 spell so they can actually use it, or can they choose a higher level spell and hope they live long enough to take advantage of it?
In the Dungeon World core book it does not expressly state a level restriction on the bonus spell for human clerics and wizards from the wizard or cleric spell lists. It does clearly state level restrictions on known spells in the specific write-ups for the Cleric and Wizard.
That said, it does state the character can cast(Cleric p92, Wizard p146) the spell and the implication I draw from that is that this Move is intended for immediate use in the game, so the spell should be of a level the character can know and prepare. This implication is reinforced in the description of the Wizard's spellbook (p146) which refers to spells of a level which can be cast when describing starting spells, and again when describing adding spells when increasing level.
You can pick any spell you want.
Wizards are explicitly told the levels of spells they are permitted to take in other places (three first-level spells; a spell of your level or lower on advancement) If the race move was intended to be limited to first-level spells or cantrips, it would have said so. Getting knowledge of an advanced spell from a race move isn't without precedent; dwarven clerics and Words of the Unspeaking (even if limited) for example.
That said, life is cheap in Dungeon World. I wouldn't recommend going too much higher than 3rd level if you want to make use of your cross-pick.