Well, the obvious answer (to some) would be to get one that fits in the camera’s card slot. But if you’re talking about the memory size, then have a look at this table.
We would suggest getting a card that would record your normal travel time over 2 days if you’re recording at Full HD. This is simply to give you some safety margin in case you want to go back and view an incident from the previous day.
It’s also a good idea to keep a spare card handy, even if it’s only an 8GB.