The last time it was a window box, and picked to small a drill bit and had trouble getting the screws in so I took my drill bits to the hardware store to buy some more screws (dropped a couple and could not find them in the dirt) and ask which bit to use, little things in the house no big deal but I'm doing to do some bolting, screwing and drilling through two thicker layers of wood. Right now it happens to be attaching stakes on a stepladder plant stand, a screen door trellis, and also some supports to reinforce the door. Next project, who knows what I'll run into.
I have noticed if I don't choose the right size drill bit, it is hard to get the screw in and you get it so far in and the slot or plus (phillips) starts getting stripped, sometimes because I didn't drill the hole deep enough and sometimes because I used too small a bit.
Do you just eyeball it and guess? For my knitting needles and crochet hooks, I have a little template that I can poke the thing in and find the exact size. Do they have something like that for drill bits? Maybe I could use that somehow. Poke whatever I've got in there and follow with a drill bit until I find a good fit.
Maybe I answered my own question here. Buy the bolts and use the knitting needle thingy.