IMO 16x20 is big and the image has to be able to stand up to that much white space. I'm not saying it is not possible because I have some that do. Both mine and other folks. On the other hand I some that seem diminished in that size. An 11x14 mat fits them well. I say it depends on the image.
I had a prof who said "if you have a bad print make sure you frame it in an expensive custom frame so people see the framing and will hopefully forget you bought or made bad art."