A transfer trailer for loading bales of forage into a transport vehicle includes a press wall mounted on a horizontal deck to be movable between a loading position and a transfer end of the trailer, and the transfer trailer has a sizing structure disposed at the transfer end formed of a hinged door arrangement such that motive means moving the press wall along the deck of the transfer trailer causes the hinged door arrangement to engage a stack of bales loaded onto the deck of the transfer trailer to force the stack of bales into standardized dimensions prior to delivery to a transport vehicle.