Spare Tire Size

I am closing in on the time to replace my stock tires and plan on bumping up to 26" Kenda HTR’s. I occasionally carry a spare tire. For a spare can a guy get away with keeping the 25" spare or do I need to buy a 26" spare also. Seems like a 25 would be ok on the front in 2 wheel drive but not so sure about on the rear. Anybody know?