The rule of thumb here is to protect the wire. Depending upon what gauge the wire is is what you should install fusewise.
Exempli Gratis:
10 Ga should have a 25A fuse
12 Ga should have a 20A fuse
14 Ga should have a 15A fuse
16 Ga should have a 6A fuse
Any fuse size SMALLER than the rating on the wire will work if you are inclined to protect your device over the wire. Just remember that if you put one in that's too small you may end up having to change it more often than necessary.
Here\'s where I got my wire amperage ratings.