i use parts with both metric and imperial measurements, but here's the lo-down from what i use:
for both 6mm and 6.35 (1/4") pots, i recommend 7mm holes. that way, especially if using board-mounted pots, it'll be easier to get them all lined-up. the washer will cover the holes seamlessly, and they won't be too loose. SOME board-mounted pots i use have threads with a diameter of 7mm though, in which case i use 8mm.
for footswitches, using 12mm should result in a perfect fit for Carling and Alphas. that said, if you want some leeway as above, 1/2" (12.7mm) can be better.
12mm-1/2" for the usual barrel DC jacks too, though i HAVE come across smaller ones (which i think are about 11mm?), so you might wanna check which type you have.
as for the input/output jacks, i use the black plastic ones (the ones you usually see mounted to boards), which require an 11mm hole for a perfect fit. unfortunately i couldn't tell you what the regular jack hole size is off the top of my head, but i think it's about 10mm. you'll really need to check that though.
i don't know with toggle switches either, unfortunately, as none of my pedal types currently use them.
for all the above, you might wanna double-check with other posters and drill the smallest suggested size for each hole, then test to see if each part fits. you can always find this stuff on the datasheets - look for a symbol that's a circle with a line through it if you're having trouble spotting the diameter.
