I size the fuse/circuit breaker to be twice the capacity of my lighting needs including the maximum surge current when plugging it into the battery charger. I size the wire to be able to handle more current than what trips the breaker http://www.alphawire.com/PAGES/383.CFM. The fuse/circuit breaker protects the battery and wiring. If the fuse size is too close to the current draw of the lamp(s) or charging surge current, you run the possible risk of it blowing due to a short-term current surge when the lamp is turned on or the initial charging current surge (happened to me!). The filament of a halogen lamp is a short circuit until it heats up (which occurs almost instantly). As long as the wire insulation can handle the current load that doesn't trip the fuse without melting and the battery can handle the load, then you are fine with a larger versus smaller fuse/circuit breaker. I use a ten-Amp circuit breaker because I draw about 5 amps when running the lights and use a 3.0 amp charger. I use 16 gauge SXL automotive wire (polyethylene cross-linked elastomer rated to 125*C) because the insulation has a higher temperature rating and is more abrasion-resistant than PVC coated wire rated to 80*C. I want my electrical system to be bullet-proof. I've had PVC electrical harness meltdowns in the field before and it isn't a pretty sight (not to mention the odor of burning/charring PVC insulation).

As with wiring, I would get switches that can handle the current of the light and momentary short-circuit current before the fuse trips. My switches are harsh environment toggle switches that can handle the abuse of rain/sleet/gloom of night/dirt/saltspray. They are oil-spray-tight washdown switches with silicone rubber seals around the toggle and are rated for 20amps at 28VDC. Bullet-proof. I want them to survive an endo into a drainage ditch filled with icy, salty winter snow road runoff. These On-off-On toggle switches cost about $11 from McMaster Carr. Mine are mounted on a plate under the left-hand handlebar grip where I can activate them with my left index finger or thumb without taking my hand off the grip. The front one, mom-off-mom, pressed forward, activates the horn (not installed yet) and when pressed backward, cycles the headlights through 7, 10, 20, 30 and 40 watts via the Lightbrain controller. The rear toggle on-off-on operates the turn signals.

Edit: Oops, forgot the picture of the switches. Added rider's view of switches under handlebar. They are easy to flick with a finger.

You can get cheaper toggle switches from Radio Shack, but these are not designed for use in harsh environments such as dirt/rain/snow/salt spray and may not last long in the field unless you are riding in mostly fair-weather conditions. A switch rated for three-Amps or five-Amps at 12V should work fine for a single 20Watt halogen lamp which draws about 1.7 amps.