Looks to be about 54cm maybe?

http://allentown.craigslist.org/bik/2402550001.html

Vintage 1982 Trek 728 Touring Bike - $500 (Bethlehem, PA)

Date: 2011-05-25, 4:46PM EDT
Reply to: sale-rpvcp-2402550001@craigslist.org [Errors when replying to ads?]

Vintage 1982 Trek 728 Touring Bike, Mostly original, new tires, original Saddle Bag Frames, Cannondale road gear bags. Great Bike, sad to see it go.