well see what current the circuit uses, then get a
fuse a bit above that.
to measure current use a ammeter, a big one.
the wire gauge is a good indication, although remember it will be slightly overrated to stop it melting too, so get a fuse with a lower rating.
* if the circuit is working normally *...
then there is one way without knowing all the currents is to guesstimate it and get the fuses below the guess and put them in and let them blow until you reach the one that doesnt.
people manage to muck this concept up all the time though.