25's aren't really noticeably better. I was 175lbs when I finally wisened up to try out 28's. That's my ideal road tire size for healthy me. Now that I'll likely be up around 240 or more this spring, I don't think that I'll he on tires smaller than 32mm. I've used 35's on velocity deep-v model rims with zero problem.

Measure the inside of your rim.

Scroll towards the bottom, there's a section on tire width.
Tire Sizing Systems

I have a bike that can only fit 25's. It's not worth what I paid for it so I'll be putting 650b rims on so as to be able to fit a more sensible sized tire.