A typical Bosch-style relay is capable of 30 amps, which is way, way more than you would ever need for what you are planning.
All voltage on the relay circuit will be 12v (or 14.4, or whatever your cars electrical system is at the time). If those fans operate off of 2.4v max, they will burn up with 12v.
Fuse size should ALWAYS be selected to match the wire size, and never go over that figure. It's OK to fuse under the rating of the wire if the equipment it's powering doesn't pull much current. I'm not sure where you got the 10amp figure, but a simple fan will not draw even close to that amount of current, even on startup.
Got a link to the fans?