This will no doubt win the stupid post of the month award, but I'm
serious. I have a little plastic case of drill bits, some are missing
and some fit more than one slot, several are duplicates. It doesn't say
(can't see that well) what size they are on the bit, but above the slots
in the case the size is molded in there.

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.