Gladly given!
Depends on the size of the track link and how much "meat" there is for a hole, but most I've done have taken .020" wire. Exceptions: the Merk4 links from Spade Ace are fairly thin, and .015" is working fine. Friul's M18 Hellcat links need smaller wire also, probably as would most links of similar size. Friul KV tracks work great using .020", no drilling required. The SA M113 tracks I did a while ago needed .015" wire.
When I go to the hardware store, I bring my calipers and measure the wire, as it's often mis-labeled: what has a .020" label can be anywhere from .017" to .022" from most suppliers. I generally buy all they have that's between .020" and .010".
You may want to experiment with brass wire as well, as it's easier to file the ends of the clipped-off lengths flat.
Here's another wrench in the werks: Spade Ace supplies a slightly stiffer wire than Friul, and because I couldn't find my required .015" wire at my regular supplier the other day, I tried SA's wire. It actually works nicely, stiff enough to insert easily and soft enough to quickly file the ends flat.
I can't take full credit for my pool toy painting tool, I read about someone using a round cardboard tube to drape the track runs over, which makes it easy to paint the portions of the links not visible with the tracks straight. I tried it and as I rolled the tube to paint, the tracks slid off the tube unless the exact same amount of track was over-hanging the tube on each side. Am I describing this well enough?
So to prevent this from happening, I grabbed an old "pool noodle" (it's a pool floatie toy made from extruded closed-cell foam in about three foot lengths, usually about 5" in diameter) out of the garage, cut about a foot off of it and draped my track run over it. The closed-cell foam is coarse enough that the teeth and outer "cleats" on the track links prevent the runs from sliding off, allowing me to roll the "noodle" to get to the ends of the runs. Old noodles that have been exposed to sunlight for a while work best, since the outer layer of foam is breaking down, giving a more coarse surface for the tracks to "bite" into.
Works better than it should........