This is the sink we got in our new 21. Same counter space as in the 5.0. ETI will no longer install this sink due to something or other (forget now exactly - something to do with underneath installation, I think), but this will give you an idea of the size that will work. It's a bar sink. I'm including the link so you can see the size measurements.
BLANCO DIAMOND™ Bar Sink | Blanco
Whatever you decide on, just send 'em a link to the sink and the measurements and they should be able to tell you if it'll work or not.
You can always add the flip-up table at the end of the counter, which will really help. I've been using mine quite a bit when I'm fixing meals and washing up afterwards.
Good luck with your decision! Lots to choose from, huh?