You probably won't have much space if you're going to be a little widget on someone's page. 1024x768 is a safe bet, but most laptops have higher-resolution; low-end macbooks have 1280x800 default.
Anecdotally --- a number of my non-hacker friends have bought netbooks recently, and use them for quickly checking Facebook and their e-mail without having to sit at a desktop or carry a bigger heavier laptop across the room. So I wouldn't be at all surprised if your users are disproportionately likely to have 1024x600.