Do you want it to be small and portable so you can carry it around easily? If so you'll need one with a good battery life and is lightweight.
If you want to use it as a desktop replacement, and only occasionally carry it, go for a big as screen as you can. What do you want to do with it?
Not too bothered about carrying it about, it will be used around the house more than anything. Would still like a good battery life though. I am getting rid of the desk top as it is running really really slow and I think it's on the way out. The laptop will be used with a wireless network along with the two my family have. Will use it for the usual stuff, internet, e-mail, gaming, films, etc.