In general, smaller laptops have less powerful CPUs, and in photo editing, CPU speed is everything. So a 12" screen with a fast CPU, lots of memory and a big hard disk, is probably not realistic. A 14" screen is probably as small as you can go.
My experience is that Dell, Toshiba, and Lenovo (formerly IBM's PC division) make the best, most reliable products, though I've heard some impressive anecdotes about the durability of Panasonic's Toughbooks. Since you'll be usingthis on the road,you may want too take a look. [ http://www.panasonic.com/business/to...k-products.asp