There's no minimum size per se, but profile images (and contact images) should have a 1:1 ratio, width to height. The ones we use for our GAL entries are 175x175 -- and they're the same ones we used since at least SmarterMail 10.x. That said, we'll look into modifying the ratio requirement in both areas.
Derek Curtis
COO
SmarterTools Inc.
www.smartertools.com