You have two choices here: 1) determine what is connected to each fuse position and determine what the total current draw is for each circuit, or 2) check the wire size at the load side of each fuse position. Chances are it is 16 gauge. If so, a 10 amp fuse would be appropriate. Understand those fuses protect the run of wire between the fuse block and the devices. If a device blows a fuse that device is likely toast so the fuse does not protect the device.
