Fuses are based on the thickness of the wiring. Thinner wires need smaller fuses.
To figure out if your wiring is thick enough, add up the wattage of all your electrics and then divide by 14 (14 is an approximate operating voltage).
Since Watts = Amps x Volts, we can do a bit of algebra and rearrange it to Amps = Watts / Volts. There is also a relationship between Ohms and Amps and Volts known as Ohms law. This lets you calculate the Amps, Volts, or resistance in a circuit if you know two of the three items.
Anyway... 10W bulb will pull .7A at 14V. Because 10 / 14 = ~.7.
If all you're running is headlight, tail light, and ignition coils, the calculations shouldn't be too bad.