There doesn't seem to be anyway to screw it up. Worst case scenario seems to be having to restore your aTV to factory settings.
As for space, it'll install from a 512mb memory key, so I can't imagine it'll take up 5gb.
By "slow it down", do you mean slowing down Apple's OS? If so, then no. Boxee isn't running when you're using the aTV OS, so it has no effect on it (least not any that I've ever noticed). Boxee itself lags every now and then, but it's beta so I'm not bothered by it.