RE: what size drill bit for 1/4 20 tap?
you'd be suprized how small a drill will work if tapping wood. You can get away with a 1/8 inch drill for a 1/4-20 tap in a pinch. Its a bit harder to tap out the hole... you have to back out and clear the chips a lot.
One time, I actually drilled and tapped in one step. It worked but it was very slow.
A good tap and die set will list different pilot drill sizes for different materials to get the best threads. When you are cutting threads in soft metal (brass and similar), part of the metal bulges in toward center, so you don't want quite as large a hole diameter as when doing steel or cast iron.