tracing size on pcb

[SUPER VELCROBOY]

how thick should it be assuming 5-18 V application?

[rmo]

Voltage doesn't matter so much, it's all about current. This website has a calculator that'll tell you how wide your traces need to be for a given current.

http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/

Your pcb's are probably 1/2 or 1 oz copper rather than the default 2 oz.

[R.G.]

For effects circuits, you almost never need to select trace width on the basis of voltage or current. The voltages and currents are so small for signals that you should select on the basis of who will etch the board and who will assemble it.

This is covered in "PCB Layout for Musical Effects".

[Paul Perry (Frostwave)]

One problem with huge fills of copper, is that it can be hard to solder to a point in it, because the big area draws the heat away so efficiently.
(I guess that is why you see those little 'target' looking things sometimes around a hole, the crosshairs let the signal thru while the circle bits stop the heat flow).

[R.G.]

Yep - that's it exactly. The circle bits are called "thermal reliefs" for that reason.