Named as a portmanteau of the letters of the states that occupy it , it is almost 300 by 100 km or about 180 by 60 miles , and is bordered by the Chesapeake Bay on the west , and the Delaware River , Delaware Bay , and Atlantic Ocean on the east . The peninsula is almost 180 by 60 miles ( 300 by 100 km ) , and is bordered by the Chesapeake Bay on the west , and the Delaware River , Delaware Bay , and Atlantic Ocean on the east .