I think it's very much personal preference, @penelopejane . For shower use, my choice is the half-thickness bars I described earlier (split a 1 1/4" thick bar into two 5/8" thick bars). A broad surface lets me scrub the bar on my wash cloth and quickly build the lather.
For use at the sink, I like a smaller, fatter bar as you describe, because it's nice to be able to rotate the bar between my palms for washing my hands. A bigger, flat bar is harder to hold onto.
For use at the sink, I like a smaller, fatter bar as you describe, because it's nice to be able to rotate the bar between my palms for washing my hands. A bigger, flat bar is harder to hold onto.