There's a fellow named Ron Grant who, several years back, rode from Florida to Costa Rica on an old Yamaha SR250 'Exciter'. He said the only time he felt the bike was "too small" was in the United States; once he got into Mexico the bike was plenty fast for the roads. You can find his trip journal with some pictures floating around on the WWW somewhere.
He only spent a week or so in Mexico, but I think he took two or three months to visit through all the various Central American countries except Panama. At the end of the trip he sold the SR250 in CR for more than he paid for it in Florida!
Bruce Clarke --- 2011 Kawasaki Ninja 250R (black)