This is kind of intriguing... Not that the 720 is particularly rare bike- but there were only 8,000 or so made over the course of 4 years more than 35 years ago. They're around- but I'm intrigued by a gaggle of them hanging around out there.

There was a hybrid bike that was given the model name of 720 in the early 90s- a completely different bike, a decent bike- but not 'flagship class' like the early/mid 80s 720. If this is the hybrid 720- then frame sizing gets a little goofy with the higher bottom bracket and sloping top tube.

You might wish to post pix of a bike similar to the bikes you're looking at (or the bikes you're looking at)- just to make sure people can give you more accurate information.

This is my 1985 Trek 720. It's running 700 x 35 tires with SKS P45 Chromoplastic fenders. Larger tires will not fit with fenders, but you might be able to get a 38 under the fork crown if you try.

IMG_2377 by Dave The Golden Boy, on Flickr