I currently have a 19" Princeton EO90 (18" vis) at home and at the office I have an IBM P201 and a P260, both 21" trinitrons (I'd
guess 20" vis). All are running at 1600x1200@75hz (85 for the P260). To be honest, I just don't notice much difference in
sheer size. I used to have a 22" P202 instead of the P260; I -could- notice a size difference between the EO90 and P202 but
that might have been more because the P202 seemed to have a slightly wider aspect ratio than the other monitors.
The thing I -do- notice with my current monitors is their relative clarity. Both the P201 and EO90 are relatively old monitors
and their sharpness is roughly the same even though the P201 is a trinitron and the EO90 is shadow mask. The P260 is
far sharper...in fact, when I first got it the increased sharpness was bothersome. But now, I wouldn't even consider buying
a new monitor that doesn't have at least this level of sharpness.
So...19" or 21"...they're pretty much the same sizewise IMO. If your budget allows you to buy a very sharp 19" monitor but limits
you to buying a less-sharp 21", definitely go with the 19". No question.