I use the 2.5 mm and the 6 mm (roughly 1/8" and 1/4"); 1/8" on the solder tabs on pots/toggle switches, and 1/4" on the lugs on the jacks (provided you're using open frame). I bought all of mine from musikding when I was overseas, they have an assortment pack with case for under $3, a ridiculously good deal, especially if you build your own cables, as you can use the bigger sizes on those.