I have year model 2000, which had a threaded headset (i.e. slightly different fork), but I believe the recent Treks 520s have about the same characteristics. If that's so:
- you won't have problems using a 700x37 tire, maybe even a 700x42 tire on the rear wheel;
- a 700x35-36 is about the largest size that fits on the front wheel if you use fenders. I once tried a Continental Top Touring 700x37 in front; the wheel could spin, but barely.