What size fuse do I need?

This is a very good question that people do not ask often enough!
Lets try to answer this by using an example.
I have a switch panel with five 16 amp switches on it. The switch panel has a single fuse fitted with a 15 amp fuse.
On one of the switches I have a LED Map lamp drawing 0.5 amps.
The next switch has my LED Nav lamps drawing 0.5 amps
The next switch has my VHF radio which is 5 amps
My cabin LED lights are 1 amp on the next switch.
And my Wheel house LED lamps take 0.5 amps.

The 15 amp fuse supplied on the panel by Dragon is there to protect the switches. But we are only using 7.5 amps so our 15 amp fuse is far too much for the job.
Lots of people would be happy with this as it gives is a good 'safety margin'. 
This is the wrong way to look a fuses. If a fuse blows that is a good thing - inconvenient but good.
In this case we should fit a 7.5 amp fuse (if we can get one) and it would not blow! Fuses have a 10% to 15% margin built into them and the manufacturer of all out electrical goods will have been given a maximum value so we probably will not be using 7.5 amps with everything on at the same time. 
The real thing that we want to prevent is fire! Electrical fire will happen when a electrical device or its wiring 'burns out'. So your fuse needs to be smaller than the burn out value of the device or wiring ( this is why your Dragon panel will have 25 amp wire and 15 amp fuse for 16 amp switches).

So in the case above even 7.5 amp fuse is not small enough. Our Map lamp above rated at 0.5 amp will probably only have 3 amp wiring running to it. So if the lamp burns out or the wire rubs through and short circuits, the wire will burn out before the fuse blows.

Think about your home circuits you have:
15 amp fuses serving many 13 amp sockets (UK values).
Each device will have its own fuse to protect it and its wiring.

So some in line fuse holders with one amp fuses sound like a good idea.