My rule of thumb is that 28mm is the widest tire that will fit between caliper brakes.
I have a Bridgestone RB-2 with 105 brake calipers, Aerohead rims and Continental Gatorskin 28mm tires. Even if I minimize the tension on the brake cable and open the quick release, the tires will just squeeze between the brake pads.
Incidentally, the crushed limestone KATY Trail is only 2 miles from my house. The Bridgestone with it's 28mm tires is my bike of choice for riding on the KATY.