I am currently building my second location so you may get better input from those more experienced but the criteria we used was:

income level of area (middle to upper, bigger units - lower to middle, smaller units), also helps determine how much climate controlled units vs conventional

flexibility - we built 10x10 back to back with 10x15 so we could create larger units if needed by taking the wall out between the two, we then built larger units in phase two because we saw the market wanted it

provide as many sizes as can while maximizing space

also consider how one unit mix may produce more income than another unit mix - maximize income

allow the building supplier to provide suggestions/drawings, then modify

if building climate, build big with hallways end to end, get more rentable space, elimates driveways between buildings

I am sure there is much more to consider but this should help start the thought process.