Width or diagonal? We've been using ViewZ 19RTC monitors. At 19" diagonal, they are the the largest we've found in 4:3 since CRT monitors were basically discontinued and they provide better analog images than any CRT we've seen - even 21"-22". I don't think you'll find anything better in any screen size.
This Samsung is about the only thing I could find (214T - 21.3", 1600x1200)
I think that's discontinued. At least it doesn't appear on Samsung's CCTV website. The largest 4:3 monitors there are again 19".
According to Newegg, "This item is currently out of stock and it may or may not be restocked."
We have had good luck with the following:
http://www.everfocus.com/product.cfm?productid=1662 (22") in the menu, you select CV which changes the display from 16:9 to 4:3 filling to the edge of the screen on the monitor, which fulfills most needs for our customers.