people grossly overstate the poor condition of nyc roads. it's nothing more than minor discomfort if your a heavier rider like me. 23's are fine. they do a good job planning the route to keep it on good surfaces and as someone else mentioned, do the 75 mile option. this is as flat of a ride as you will ever do. the full century has a few small hills in the BX, people always overstate thosehills because manhattan is so flat but they really aren't that tough at all.
I did the 75 last year because I do a ton of riding in the BX already and I wasn't going to get to see anything new. here is a link to my ride last year, you can see it's very flat, only about 3000 feet of climbing over the span of 70+miles.
http://runkeeper.com/user/cpfitness/activity/16312683