gleaner made 20 inch heads way back in the 60s and early, early 70s, A,C, and G and maybe F series. pretty clunky and high maintenance to try and run on a N series. we have a A420 and a C620, and a C620 with a factory adapter kit for a G.
I know the newer huggers can go to 22 inch, and possibly 20. Deere will definitely go to 20, and you can run one with a adapter plate from Bish.
I think 22s are more common than 20s if you have not invested in the equipment yet, it may be better to aim for 22s instead.
Our farm purchases a combine soley on its ability to do an excellent job in all of our crops, while being fuel efficient, reliable, ruggedly built, and easy to repair. That is why we choose a Gleaner.