"The issue is clearance, Clarence." The larger diameter wheel must still be installed so the blower wheel fits rather snugly into the housing yet not rubbing.
I had the same problem with my external exhaust vent for my kitchen. The clearances were too tight to go larger, so I went smaller and fabricated a plastic ring out of an old frisbee to fill the gap and provide proper tolerances for good air flow.
Here's a typical installation - note the tight clearance at the round ends of the fan cage.... this is what creates the pressure differential when the fan spins. It is only because HVAC is low pressure air that you can get away with any clearance at all.
Generally smaller is the only direction you can go in due to these tight clearances - the critical match is right where the ends of the squirrel cage interact with the housing. Even a small change in this clearance (for example, with a smaller squirrel cage) will require the fabrication of some kind of rigid gasket to make the gap correct.
May I ask what is wrong with your old one fan wheel? It might be best to have it straightened or repaired if necessary vs replacing with the wrong size. They last pretty much forever unless someone has destroyed it trying to pull it from the motor.
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