In 50 years of driving and several 100,000s of miles, I've used a spare tire 2 times (both on the same car!) That car has a mini spare (a regular tire & wheel, factory equipment) that was at least 2" shorter then the 4 regular tires. I don't carry any spare tires in 2 of my 3 vehicles (the car with a spare is the same car I've used the spare on, believe it or not). When its shot, I will not replace it (I haven't checked it in a while, its probably flat, but I'm not sure it would come down out of the hole its stuck in anymore).

Given the difference between the factory spare and the regular tire on most modern stuff, and the concept the spare is only suppose to get you someplace to get the tire repaired or replace, I doubt the difference in size is that big of a deal if you get the tire fixed as soon as you can. If the difference is big, drive slowly, or have it towed in.

Back in the 70s Challenger days, the inflatable spare was 2" - 3" shorter then the original tires on the car and the factory thought that was OK back then.