This is obviously a labour of love for James, unfortunately love doesn't pay the mortgage or put food on the table. Four main things have to be considered: Storage (pre & post production), manufacture, packaging, despatch. To produce multiple sizes would increase costs that must be reflected in pricing which could possibly make the product financially unviable in an already very competative and saturated market. If machining allows, I would probably go for 70g flat slabs, easy to store taking up less space, postage & delivery easier, one size makes production more efficient and cost effective, in theory the smaller size means it will be used quicker so more soap bought thus better turnover. The consumer could keep the soap as it is, re-use a plastic tub or go to the local charity shop, supermarket etc and pick up a china, earthenwear or whatever bowl for pennies, use your imagination. This is a small business whose product probably has a small mark up so its all about efficiency. Efficiency equates to better production and consequently, better profit. If the chap doesn't make any money for his hard work then what's the point? The main downside I can think of would be marketing. Preaching to the converted like us is easy, selling it to Joe Public who wants pretty labels and convenient packing & dispensing may be more difficult.
So there you go, that's my tuppence worth!!
(And off topic for a sec, loving the Bay Rum soap! Had a shave with it this morning!!)