I'm running 28s on my road bike, a LeMond Zurich. That's as big as you can fit without the tire rubbing the top of the fork. Sometimes I have to let out some air to get the tire past the brakes.

28 is a nice width for a road bike. 25 is good, too.

I like the hold on the road a wider tire provides. Smoother ride, too, and fewer flats.

I'll never go back to 23-thin tires. I'm surprised that is what most riders use.

My commuter is running a 32 on the front and a 35 on the rear, because I also use that bike for grocery shopping, and sometimes it is carrying a load.