I use duals, my main a 24" that holds the preview window.  In the pose room, I have it at 1400x1275, which matches the ratio for the widescreen monitor (I render everything now at widescreen and trim later in Photoshop).  It's smaller in the material room, and not with any real standard aspect ratio because I move some of the palettes over from the second monitor so my material palette occupies all of the 20" monitor.  In the cloth room, I enlarged it from the default just so I can see details of the simulation better, but kept all those controls on the same screen so it's maybe 800x800.  Hair room is about the same.  Face room I don't use, so I haven't bothered to chnage anything there.