Fenders and trek 520

bentupright said:

Is 35c the biggest that I can fit on this bike tire-wise, and can fenders fit WITH the stock 35c's (one source showed fenders with 32c tires, that made me wonder what was up with fender restrictions)?

It depends on which fenders (and how they are mounted) and which 35c tires. That combination can work, but you should be sure before you commit to the particular combination. Panaracer Pasela 700CX35 with Gilles BERTHOUD stainless steel fenders (slightly modified) will clear on at least the Trek 520 that I helped with.
The seat stay bridge and the chain stay/fender clerance near the bottom bracket are the keys.

bentupright said:

Is 35c the biggest that I can fit on this bike tire-wise, and can fenders fit WITH the stock 35c's (one source showed fenders with 32c tires, that made me wonder what was up with fender restrictions)?

I've had a couple of different sets of fenders on my 520 with 35-37 mm tires. I had a set of 35 mm fenders first, but I had to make a couple of little aluminum extenders to get it to clear on the front fender. Then I got a set of 50mm fenders and they go right on with no problem. You should be able to get bigger tires on there with no sweat.