Hi Michael,
I do not sell, but do give plenty of my soap away! When I label I will always weigh before and label with that amount... so if a soap weighs 3.28oz I would list it as 3.2 or if I wanted to sell at a set measurement I would try to perfect my cuts so they all came out at that weight or a little over. I would never list a soap as 3.5oz if it was 3.2oz. I think the best practice is to always give the customer the benifit of the doubt.
As for the sizes, why make it too complicated? If you are not going to sell by the oz (a customer asks for 2oz of this and 5 oz of that) then have a small bar say 2 oz and a large bar of 4 or 5 oz. I would think of making them different styles... round for the small and rectangle for the larger. Just my thoughts on helping you out. More from the buyers aspect though
Susie