I use a 1/4 inch bolt or threaded rod for my nuts. The string height can be adjusted by how deep of a groove you cut for it. Smaller would work fine also, just cut a smaller slot.
I don't know if there is a "standard" size. If there is, I don't follow it. It depends on what you want to accomplish. For strictly slide playing, you'll want larger bolts for higher action. For fingering you'll need smaller. It also depends on your frets, whether or not you have a fret board and other factors. I have a variety of sizes in my tool box, separated by what I'm doing on my build. After a while, you'll know immediately which one to grab based on your build.
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