The Islanders’ proposal to build a new arena at Belmont Park was selected as the winning bid, according to people familiar with the situation. A news conference is scheduled for Wednesday morning at the racetrack in which Gov. Andrew M. Cuomo is expected to be on hand. The Islanders’ bid includes an 18,000-seat, year-round arena that would host 150 events annually as well as 435,000-square feet of space for retail, a hotel with 200 to 250 rooms and a 10,000-square foot “innovation center” that would be developed with resident input. The Islanders partners in the development include Sterling Project Development, a real estate firm run by the Mets’ Wilpon family, and Oak View Group, an arena development company partially funded by Madison Square Garden. The proposal was selected over New York City FC, a professional soccer team partially owned by the Yankees. That bid called for a 26,000-seat open-air stadium in addition to 400,000 square-feet of space for retail, a 5.2-acre community park and a 2-acre soccer facility.