Use a fuse size that will protect the wire. To find the proper size go to:
http://www.powerstream.com/Wire_Size.htm
Measure the conductor or see if the size is printed along the wire. Use this size/AWG to find the max chassis current/amps in the table. Your max fuse size should be at or less than this value. Another point to consider is that most isolation relays are controlled by the circuit they reproduce and have battery power connected to the input contacts. You should trace this wire back to the battery to make sure a fuse is installed as close to the battery as possible. If the fuse is located somewhere at the back of the bike it will protect the trailer from damage but a short on the battery side of the fuse is an uncontrolled short that can melt wires, start fires, and cause the battery to explode. At this point you can't stop it by turning off the key and it will continue until an open occures or you have no more voltage to produce current.
You can always use a smaller fuse but never more than the max current for that size wire.
