There are only so many places one can hang a framed 20 x 30 or larger print. As photographically oriented as I am, for example, I have only 4 of them that size anywhere in my house, and only one is a portrat (of my son). So, as you alluded, the market is limited for that size.
Note: Selling prints is really only a sideline for me, so take this as only a small datapoint... What I *have* had sell halfway decently is a 24 x 36 fine art print on canvas (I have a moon image that is selling by word of mouth amongst the astronaut set). Winkflash actually makes such prints at a pretty reasonable price.
-Noel