Tire sizes

I recently bought a new HTT, as my first camping trailer. All in all I was less that happy with the pickup and delivery process. As we did the walkthrough and the mechanic showing us how to use all hte equipment, we had multple issues with the with items that were supposed to be on the unit, not being there. Most notably a 19 inch TV, and a spare tire.

We had actually traded an outside grill that was part of an option package for the spare, but the work order to swap them 'never made it from the sales office to the shop floor'. When we noted this, and pulled out the sales contract, to show their mistake, another mechanice was called out, and brought a spare tire and carrier, and mounted it up t the storage bummer. We thought issues resolved, no big deal.

Yesterday, when we had family come over, one of them made a comment that the spare did not match the four ain wheels. I retpred that yes the spare is a steel rim, not AL. He replied, its also a 15, while the others are 14's. I take a closer look, and sure enough:

Mains ST205/75R14
Spare ST205/75R15

Now I don't know enough about tires to know if this is critical. does only the 205/75 need to match, or is the 14 vs 15 critical as well?