Don't forget more importantly than the expected amp draw and switch capacity, the wiring gauge and subsequent amp capacity is the other biggest factor in fuse size. The entire purpose of a fuse is to be the sacrificial component in the entire circuit so that it heats and burns first, not the wiring or the controls or powered source. Make sure your wiring is not a lower amperage rating than the fuse you choose.

A relay is a current reducer, a fuse is a circuit fail safe.

BTW, let me know if you need anything. I have spools of automotive wiring, fuses, leads, switches, etc. You're welcome to stop by and grab what you need.