Lex Jenkins , Sep 06, 2008; 05:48 a.m.
In my area (Fort Worth, Texas), the Amon Carter Museum has a large gallery of photographs dating back to the 19th century, and hosts traveling exhibits. I haven't noticed any particular trend in sizes. Contact prints from large format are common, so the typical sizes are seen - 4x5, 5x7, 8x10, etc. Enlargements run the gamut from a few inches on up.
From recent decades 16x20 seems to be a popular size for more contemporary prints in museums and local galleries, regardless of film format. Prints from Avedon's "In the American West" exhibit included virtually life size portraits. Elsewhere, in a Dallas gallery, I've seen several Michael Kenna prints, most of which were approximately 8"x8", framed with vertically oriented mounts.
I'm attending a local gallery crawl tomorrow. I'll update this thread if I spot any different trends.