I work with 3 monitors, two 27 inchers (Samsung and a new Asus IPS) and a 20 inch Apple monitor. But find other than using the new Asus for reference images, the 20 inch can handle most UIs. I also primarily used Photoshop, but more and more these days I have been using Manga Studio 5, and sometimes Mischief, which gives you an infinite canvas... so UIs don't matter in the latter.
Edit: forgot to mention I use an Intuos 4 Medium which has a 6 x 9 1/4 surface to work in... but I started on an Intuos 2 back in 2003. I've dabbled in Illustrator but my wife is the vector artist not me, she can even do that crazy vector mesh coloring... I just don't have the patience. lol