Heck I'm kinda fat & slow myself I guess, but I became a fan of 32mm plus the last time I caught a good road crack at speed with 28's.
I believe Jan Heine might dispute narrower tires, but heck there's a lot of discussion on it, so if ya have the time & $ try 'em on your own.
Jan's take on it is here - requires acrobat to read it.
For my 700c bike I'm currently running a worn pair of 35/37mm Vittoria Rando's that have served me well. I liked 'em a bit better than the similarly sized conti contacts that I'll probably start using when those ones get too worn (I didn't dislike 'em I just liked the rando's a bit better). After that I'll reassess what's out there & may get something fancier.