Well, there's netbooks, which are small, relatively cheap, but underpowered. I've got an eee PC 1001PX, which is fine for browsing and email, but the screen's too low-res for much else. Bear in mind that I've got 3 monitors on my work PC, and that my "much else" is Visual Studio, though...

Then there's "ultrabooks", which are small, relatively powerful, but quite expensive. I'm tempted, but I'm waiting until the new year.

Then there's the 13-15 inch laptops, which are a good compromise on screen size, and they're fairly powerful, but they tend to be a bit heavy. I've got a Samsung SF310 (13"), which is nice, but a bit heavy for my taste.

Of course, if one is happy to carry around a heavy laptop (maybe you're driving), then there's the desktop replacement laptops, which tend to be quite large, very powerful, and relatively expensive. Useful if you're visiting customers and you need to either work there, or demonstrate stuff.