Most of the touring bikes back then used 27x1 1/4" rims/tires. If you find one with a 700C someone probably changed the wheels or had something built. If you find a frame without canti bosses you can put whatever you like on it, 650b (check clearances and steering) or 700c or 27". If you get one with Canti's it will limit you to 700c or 27" unless you want to modify the frame or have it modified.
Get the frame then worry about tire size. Decide if you want fenders or not, as that will influence the tire size also.
The 6xx aren't automatically sport touring frames either. If you find a frame , check the year and then the old catalogs to see what the frame specs are, one year the trek 620 had the same chainstay length as the 720. most others years they were shorter. The 520's usually had short chainstays compared to full touring frames. The later 600/610 series make good 650b conversion frames also.