It looks like maybe a 48cm, measure from center of crank to top of seat post tube to confirm or look up s# at vintage trek. Really small or large sizes require the similar sized rider which means this one may be hard to sell as these sizes are outliers. The steel bikes of this vintage are more widely sought, so that may hurt value too. Roughly it may be worth anywhere from 150-200 based on whether you put new tires on it and if you have a good local market to sell into.