Well I'd generalise and just say there is more space on a 23" screen than a 20" screen (obviously), and for design that is a good thing IMHO. You can look at more material at one time, have space for many toolbars and palettes etc. If you gave me the option and cost wasn't an issue, I'd take the 23" every tome (as long as they were equally high quality monitors and i had desk space for the 23").
Whether you can show an A4 spread of not depends on how you work and what you are working on. I know my 19" , 4:3, work-provided monitor is deeply irritating when i have to work on layout in Indesign. On the other hand, some people seem to like to work at under 100% zoom for overview then go right in for details, others have their own practices. For instance, I can pretty much get two full pages of text on my 1440x900 screen (both the one in my MBP and the external one), which helps me a lot as i write for my job.