Also agree with rwyoung.
I also have only 11x14 heat press. For 16x20 prints I sandwich one at a time between 2 sheets of 4-ply acid free board and set the dial to about 175 degrees and press sometimes for up to a full minute. I first heat press the middle area and then go around and press each 1/4 section which will overlap each other.
With these, as with smaller sizes, I place the sandwich right out of the press under a 21 x 18 x 1/2 inch sheet of glass. This is quite heavy enough for a single print sandwich. While this is cooling under the weight, I continue to heat press the next print. Then take the previous one from under the glass and place the next one from the press under the glass. This sets up a sort of assembly line - while one is cooling, one is heating and so on and so on.
If I end up with several prints, I'll stack then together under the glass with books on top for hours or overnight.