I think that there have been recumbents built with every wheel size and
combination available. As far as I can see, the main considerations
with wheel size and the SWB configuration has to do with rolling
resistance and seat height.

When comparing a SWB with a 700c rear wheel to one with a 650c rear
wheel, the 650c wheel gives you similar rolling resistance (given
similar tires) to a 700c wheel, with the advantage of being a little
closer to the ground. This helps in handling the bike when stopped, and
a lower center of gravity in motion.

So a 650c wheel gives you most of the advantages of a 700c wheel, and
puts your foot (and butt) closer to the ground. If you're building, you
can do anything you like. I think the commercial 'bent builders have
come to the consensus that the 650c wheel does what they need when they
spec a skinny tire, which is why you haven't seen much else.

Jim