Some things to consider when choosing a soap size:
1. If choosing from commercially available molds, whether you want to buy the accessories like silicone inserts/dividing bars/cutters that are specifically made to go with those molds like BB's, Essential Depot, etc. rather than making your own.
2. What size fits best on your shower's existing drain tray/soap tray. (My shelf is rectangular so my big 3.5" x 3.5" square bars hang over and fall off.)
3. What thickness provides the best compromise between longevity in the shower, your desired cure time, and the size of your grip.
4. What kind of packaging you will use and whether it requires uniformity or not i.e. cigar bands (forgiving on LxWxH), half boxes (forgiving on H), or boxes (not very forgiving).
5. How much you typically shave off, if you use a planer to clean up soap.