I had a 59cm Romulus with caliper brakes for a little over a year. I ran 32s without fenders and when I ran full aluminum fenders I downsized to 28mm tires. In my opinion that's the "proper" setup with caliper brakes. With cantilever brakes, there's got to be a little more room. The generally recommended rule of thumb when brakes are already out of the way is if you've got 15mm above the tire to the fork crown, 15mm above the tire to the chain stay bridge, and 15mm above the tire to the seat stay bridge, then you've got room for metal fenders. LOTS of people go with less headroom than that.
Those are three objective measurements:
1. how many mm from the top of the tire to the front edge of the fork crown?
2. how many mm from the top of the tire to the chain stay bridge?
3. how many mm from the top of the tire to the seat stay bridge?
Whatever the smallest number, that should tell you what the "right" size tire would be, and if you want to push it a couple mm, that'll give you a good estimate for what that would look like. In general, plastic fenders can take up slightly less space than metal fenders because the L-brackets are flat pop riveted to the fender.
Bill Lindsay
El Cerrito, CA