The idea behind a fuse is to allow the circut to operate normally, but if it starts drawing too much power, shut it off.  If the fuse is too small, it will blow when nothing is wrong.  I it's too big, it doesn't protect the circuit like it should and you could fry componets or wires.

If there are no problems and everything is working correctly, you could do it by trial and error . . . start with small fuses and keep replacing them with bigger ones a step at a time until they quit blowing . . . but if you have a short, you'll eventually get to a fuse that is no longer the weak link . . . then stuff fries!

I would get a wiring diagram for a late 60's/early 70's bug and look at the size fuses they use for each circuit.  This should get yoiu in the ball park.

Everything uses a blue one. Except for the headlights, they are the yellow ones I think.

Maybe that's something else 

Do you know about the red and white fuses ? They all have different values. Sure you can use all heavy amp fuses until a little gremlin pops up on a  component  that draws only 8 amps. be fore it "blows".  Its the glowing part followed by smoke then fire that sort of gets  my attention every time.

I think if you find out the wattage of the headlight, you can calculate the amperage draw, which will let you size the wire and the fuse for that circuit. As for the other items, I'd look at comparable setups and review thinks like what fuse is used for a horn and take into account what else is on the circuit. Then you put in a fuse and if it blows, step up. But remember, the fuse is meant to blow before a wire carries more than it's maximum allowed amperage, so don't overload a wire!