So how big a fuse do I need?
A clamp (clamp-on) amperemeter is a fine tool, let's one check drain or charge with system in use, without disconnecting anything. Even the most inexpensive ones are more than accurate enough for car\bike work, and they work as basic multimeter as well, with volts & ohm.
Check charging, current drain,
reading the current drain (A) under normal load = easily choose a suitable fuse value, etc.
Picture2, current shown = main lights on (6v system), brakelight adds 3 A, + horn some more A's, so in my case a 15 Amp battery fuse would be right (have a 10 A that have been working so far).
Also fine to check charging current with dynamo running, help trace battery discharging causes etc.
Basically does the same kind of job as the bike's own ammeter, but as it can be clamped around any cable anywhere it's very useful.