Originally Posted by davidsh
For headphones, probably somewhere between 20-26 AWG. Many like stuff like the canare star quad cables for making cheap cables.
This is a good rule of thumb. Keep in mind that the number has an inverse relationship with the actual wire size. So a 20 AWG wire has a larger diameter than 26 AWG.
Another note: you want stranded wire, which will have more flexibility than solid wire (just to make sure that you buy/ask for the right stuff