I use 2 17's at work and I love it. Stretching the desktop over 2 monitors is extremely useful.
i've tried 19's, and yeah, they're bigger, but they just really take up more real estate. For the most part, both 17"s and 19"s only run in 1280x1024. To really appreciate larger monitors, I'd need both in 1600x1200. (Which is something I really don't get. Why is it that they have 15" monitors in laptops running 1600x1200, but almost every desktop 15" LCD has a max resolution of 1024x768?)