The short reach brakes on most road bikes will limit you to a max size of 700 x 28. Some forks and frames are tighter still, but most bikes will allow a 700 x 25.
I've been between 250 and 210 lbs during the last few years. I started on 700 x 28 but now use 700 x 25 without worry.
I also started with a Cyclocross bike, these are drop-bar road bikes with brake systems and clearence for larger tires, 700 x 35 or larger. Cyclocross bikes are often tougher than road bikes, since thay are raced off-road.
You might visit this forum here at bike forums for more info:
http://www.bikeforums.net/forumdispl...-(200-lb-91-kg)