The viewable area on a LCD is bigger then a CRT so it is hard to compare sizes. Also the Apple 20" and 23" are wide aspect screens unlike that of the CRT monitors.
The Powerbook that I just ordered is also a wide aspect screen and I also plan on plugging in my Apple 22" CRT display when I'm at my office. I love being able to keep my pallets on one screen and work on the other screen. I do that now with my two 22" CRT monitors and actually with the Powerbook's screen being wide aspect I will end up with even more screen real estate then what I have now.
As with most things in life bigger is better.
