There isn't a chart to say which yarn thickness to use in which cylinder in the sock machine. Gearheart put a list together but it was "yarn weights" like 12/2 and 16/3 and they didn't say which scale to use (woolen, worsted, ?cotton?) which would effect the end result a fair bit.
(For those who don't know sock machines -- each cylinder has the same diameter, about 4.5", but a different number of slots for the needles.)
On the 100 needle cylinder, I use baby yarns (very fine, sub-fingering to fingering) or Opal. Regia's a bit too "thick". Cranking around 100 stitches is alot of work, too, LOL! But when you want a bigger diameter ankle, it really can fill the bill.
On an 80, I usually use Opal, though Regia's nice and cushy too. Occasionally I'll use a finer wool (the ones on my feet now, for example) -- I got a very nice wool I dyed in a class once, it was a brown ashland bay trading wool (maybe a wensleydale?) and wears like iron, though comfy and warm! Nice because it's thin enough to fit in my tighter shoes.
On a 72, I'll use Regia or some of the all-wool fingering weights (which are slightly poufier than Regia)
On a 60, Opal's a bit "thin", Regia is okay but a poufier wool or a sportweight is nice. I can use a DK weight, but usually reach for the large-eye needles then or suffer alot of yarn-splitting issues, especially on the closer to worsted weight yarns.
On a 54, I'm definitely looking at DK weights or better, and usually making mittens to felt into a more solid yarn once I'm done. So, I'm usually reaching for my large-eye needles (dunno if they still have them, but a few years back I think Pat Fly, Barry Travis, and maybe John Loeffelholtz (sp?) carried them)
I tried to figure out WPI's once for cylinder size but have since given it up. I'd love to hear others' recommendations on this topic too.