I just checked the genuine Garmin car charger that came with my Zumo, it says 1A max. They all use the same charger, looks the same as the one my wife has for her car Sat Nav and for the car Sat Nav I have in the Astra. I think I use a 1A fuse on the bike, but 2A will be fine if your wires are sized properly (they'd have to be really really thin not to handle 2A.) Personally I doubt the charger really pulls a whole 1A at any time.
As with Gassoon's socket, if you want to use it for something else too, like a pump, then you need to find the thing with the highest Amp requirement that you will use and use a fuse appropriate for that. Again, your wires need to be able to handle the current that the fuse is rated at. If you're only using it for a Sat Nav than 1A or 2A fuse is fine.
In the end it's all about protecting the wires, not the device that you plug into it, but there is no point in fusing larger than you need to for what you will eventually connect to the circuit. For that matter, there's no point in having grossly oversized wiring either, but having something that can handle slightly more than you plan to use it for is something I usually do (ie. 17A wire even though I don't really need more than 15A.)
