As large as you can afford but don't go for the cheap ones, you may be lucky to see the rings of Saturn but they won't be that clear.
Try to get something with an "Equitorial Mount" (motor drive is even better), if you can stretch that far, it makes life so much easier than tracking the stars yourself because by the time they're in focus they've go out of view.
The reflector ones give a better immage then the refractor and you're looking at around £150-£200 for a half decent one.
Here's one link to get you going but make sure you see the size of the thing before buying it as they do take up a bit of room!!
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