The primary consideration is that the fuse is to proctect the wiring from overheating and compronmising the insulation. So the critical factor is the wire guage, insulation heat rating, and the environment in which the wiring is installed. That is what determines the max fuse to be used on any circuit.
You are of course then free to use a lower rated fuse than what the max allowed for the circuit wiring, as long as it also provides sufficient overhead for the device demands.
Dan's recommendation of 7.5 amps should be well within the ampacity of a reasonably sized wire, provided the insulation is rated for the kinds of temps you will find in the engine compartment (also presuming the wiring you added traverses the engine compartment) - but since you haven't stated the gauge of the wiring you are using, you may or may not be ok.
Do a web search on ampacity and you will find reference tables for max amps through a circuit for the gauge wire you are using. Look for a table that factors in the insulation rating that matches the wire you are using.
Usually the design process would be to first total up the current draw of the devices on the circuit, then determine the length of the circuit, decide on the max voltage drop to be allowed, and then size the wiring to suit. If your circuit maxes out the ampacity of the wriing, you will likely find the voltage drop to be more than is desirable.
