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.
(877) 357-6278
www.smartertools.com