Many things to consider, but a rule of thumb is to have all tires the same size, for a multitude of reasons. Personally I deplore mounting a spare underneath a vehicle for a lot of reasons, access being the primary and because for a lot of people since it’s out of sight, it’s out of mind, until you need it. Then find out it has about 10lbs of air and it is dry rotted. If you are truly remote and you cut a sidewall, which cannot be repaired in the field, you might be a day or two away from the nearest tire shop, then wait for shippping, at least 24 hours. Meanwhile your driving around on a poopy old wrong sized spare tire, that will wreak havoc on steering, and unless you have open differentials will wreak havoc on your driveline. Well, these are a couple of reasons, there are many more that I’m sure others will chime in on. Good luck! Say, where are you headed?