It really depends on how you use it.
For just taking notes and writing shorter texts, a netbook is perfect. I've got an Aspire One 8.9" with a 9-cell battery. It's very lightweight and easy to carry around, the battery lasts literally all day so I don't have to scramble for a power outlet after 90 minutes, and I can leave it in standby when not using it so it boots virtually instantly. However it requires that you at least have a full size keyboard and monitor and home, if not an entire desktop system. Typing 5-10 pages on it might be okay, but not 30+. It was cheap however, so you could easily fit a low-end desktop system into the budget - you can buy no less than eleven Aspire Ones for the price of one 17" Macbook Pro....
For using the laptop as a primary system, writing longer texts or doing more serious stuff, a mid-range 15" model is required. I would prioritize ergonomics and battery life over CPU or GPU power - unless you're a gamer, any modern dual-core processor from Intel or AMD is enough and even low-end laptops come with 3GB RAM and big enough hard drives. I would only choose a 17" model if I did something that really required a big screen, like web design or photo retouching. Even then, the cheap 17" screens are probably poor quality so the money is wasted anyway. 17" systems are too heavy and bulky to easily carry around.