Anything of paper - print, watercolor painting, or pastel - needs a mat or spacers to keep it off the glass. The image touching the glass will in most cases not be good for the image. A 16 X 20 ready made frame holds a 20 X 16 image including the mat which is usually 2 to 3 inches in all the way around. If you purchase a 16 X 20 print it will have the extra white border to compensate for and allow the image to be affixed to the mat so that it doesn't slide around; the frame for the 16 X 20 image would have to be larger by 4 or 6 inches, making it a 20 X 24. Depending on the width of mat - unless you go with using spacers - the image should only cover 12 X 16 of the paper unless the buyer would be happy/satisfied with covering part of the image. And remember frames are not measured from the outside of the frame but from the inside rabbit.
Framing is a huge problem for print makers, watercolor artists and those that use pastel - you must take into account the matting and the glass.
Custom framing has never been cheap. I once sold a half sheet (16 X 22) watercolor to a lady who asked if I'd take it out of the frame - sure - I even took $10 off the price (generous, I know - and I used the frame and mat again). Later she asked if I knew how much she had paid to have it reframed and told me it was over $100 - and that was several years back.