About 7 ft wide I guess, that will give you room to put fenders over the
side(s). 3/8 line nylon should do just fine, bow, stern and spring line
should be all you need. Might go one bigger if you won't visit much or the
marina has lots of wave action that could chafe your lines, but most cleats
on small boats won't take much bigger than that.
Good luck with the boat.