I just bought a used Bianchi Campione D'Italia set up as a single speed. It hasn't arrived yet but I already know I want to enlarge the front chain ring. How large can I go? I think it's 18 in the back.

I live in the Portland Maine area and have been commuting solely by bike since June. It's nearly all flat land around here and so I thought I'd like to gear the bike somewhat high. How big can I go on the front?

thanks, jack