The fuses are only to potect the wiring. size the fuses no more than the wire capacity. The radio GPS etc will have their own internal fuses to protect them, so if theyre the only items on a circuit you can size the main fuses the same or slightly larger, even if they the same size, for example 2 amp in the GPS, and 2 amp in the fuse box, If there is a GPS fault the fuse in the GPS will go first as it will be a faster fuse. The automotive type blades are fairly slow blow and will carry double thiir rated current for a short time before they blow.
I am usually not as green as i am cabbage looking.