not sure, but I wouldn't presume any more than 16 gauge....since most of the wiring throughout the car is 16 gauge. But if you're running a power line yourself, it wouldn't hurt to use 8 gauge either- then you can also tap off of it for any other things that require a constant power
and sorry, no idea about the amps- but the fuse is more for incase of a short than anything else, really.
post up the question specifically asking people with power seats and see if they can find a fuse amperage for you
good luck