Inline fuse for switch

Hi my sister's boyfriend just got a car a corolla to be exact and he got t he neon washer things on the hood and he hooked them directly to the battery but the problem being that they dont turn off and I know how to hook them up to a switch no problem at all since I have done some research on it cause I was gonna hook up some neons but im not anymore but my question is when i run a wire from a switch to an inline fuse to the battery what size fuse should i put in the inline fuse? He has the streetglow ones in and I have 18 guage wire i used for my vtec indicator light or should i get thinner wire for him?

i would go a little bit bigger fuse like 30 amps, that way u have enough power to power anything he will have running in the car. and from the fuse run a little bit bigger wire than 18 awg from there to the accesories

so i should run the 18 from the switch to the fuse then run a biffer wire to the battery or where should I run that wire? I was gonna run the 2 postitive wires to a switch im gonna buy for him, then a wire from the switch to an inline fuse and i guess ill put to 30 watt fuse in there and then run that wire to the battery or where should I run it?

This is what he has
http://www.streetglow.com/products/anw.htm