Dual (or more) monitors is a must, IMO. I hate tabbing through things and I like everyting in front of me. I wish everything these days wasn't widescreen - 4x3 monitors rule for the larger vertical space (especially for track view). For me, widescreen monitors are better for displaying a bunch of channel strips. Regarding size/resolution, I think it depends on your eyesight and what you're comfortable with.
I'm seriously considering upgrading my DAW to Win8 (or 8.1) and getting a pair of 22" or 23" 1080p touchscreen monitors. Definitely not needed, but a cool toy.
Right now, I'm using 3 monitors, a 17" 4x3 LCD, a 22" LCD (1680x1050), and a 19" 4x3 LCD (portrait mode) on my DAW. If I went to the touchscreen monitors, I'd probably mount the smaller ones above on an arm to display things like plugins or maybe the time/measure counter (so I can see it from across the room).