DONALD Trump has found a new enemy: ESPN. In “Small Potatoes: Who Killed the USFL?” — a new documentary from the sports network — director Mike Tollin argues that the long-gone football league, which was established as a competitor to the NFL in 1983, folded mainly because Trump, then the owner of the New Jersey Generals, insisted that the USFL go head-to-head against the NFL and move its season from the spring to the fall. In the movie itself, Trump cuts his interview short and calls the league “small potatoes.” He told Page Six yesterday: “The documentary is third-rate, as was spring football. Football was meant to be played in the fall and I realized that early on. Tollin is a sad guy who is living in the past. He ought to get on with his life.”