I use 2 22" monitors, 1 for track view and one for mixer view (one of them is an HDTV), then I have a 19" monitor I keep the browser open on for drum loops and synths.  That works great for the way I do my recording.

For me its more about resolution than size. I have a 24 inch 1920 x 1080 and I can see the point in going any larger at that resolution unless you need to work a long way from the monitor or have poor eyesight.
 
I have considered adding a second 24" 1920 x 1080 but space is an issue for me. At some stage I may go for a 27" 2560 x 1440 but they are expensive at the moment. Someone mentioned a 29" wide screen 2560 x 1080. The issue for me would be the vertical resolution is only 1080. I feel more vertical resolution would be the most helpful thing for me. Another reason whay I may prefer a 2560 x 1440 to dual monitors. 2560 x 1440 gives you 89% of the screen realestate of dual 1920 x 1440