what size fuse should be used when going directly off the head light power wire for a small 3" neon?

I'd recommend something other than the headlight wire. The headlights draw lots of power already, and are on a relay, and if something goes wrong with the neon or the fuse, losing a headlight is probably going to get you a ticket faster than anything else.

Instead, are you close to a side marker light? Is the neon on its own relay? Do you want the neons on only with the headlights - or would just the parking lights be alright?

And now to answer your question, I'd start with a fairly small fuse, either 7.5 or 10. The light didn't come with instructions or a recommendation, I assume?

A 3" neon tube will draw a neglegable amount of power. Make doubly sure it's a 12v supply.

i want to wire it so that it goes on with the headlights and it doesnt have a relay just the light. and i dont know if its 12v it originally pluged into the cigarette lighter thing and then went to a box then to the light. i dont want the box cause it was cheap and didnt work all the time so i just want it to be directly wired to the head lights but i dont want to blow anything.

Does the box have any writing on it? It might be a transformer, and the neon might not be 12v, so you'd need something other than the just the light...

What brand is the light? Have you googled it?

yeah its got wiring. dont know what brand. got it for free from one of my ex-bosses sons. guess i will look around for other lights then. thanks for the help.