Complex sentence: His football role was expected to change when he reached college--at 270 pounds out of high school, many felt that he was simply too big to be a tailback and believed he would be best suited as a fullback.
Split into simpler sentences: His football role was expected to change when he reached college. At 270 pounds out of high school, many felt that he was too big to be a tailback and believed he would be best suited as a fullback.