Almost all road bike frames and brakes will accept 25s. 25s are a sweet spot if you ask me because you can get them past most released caliper brakes without deflation. 28+ requires deflation for most calipers even if they're released.
You can run some pretty wide tires on even the narrowest road rims as long as you keep them aired up. They might start to roll over on you in the turns if you run some 32s at low pressure on some 19mm outside dimension rims.
If you need a more precise answer, offer up rim make/model and brake type, even make/model if you'd care to.
Edit: fork make/model is typically the next problem spot. Then after that you'd have to consider the chainstay clearance.
Last edited by LesterOfPuppets; 02-26-11 at 07:34 AM.