The photo by Pat Fly from angoravalley.com shows three different gauges of needle. Each brand of machine uses it's own particular gauge.
If you scroll down to my post about Lorna's Flamingo you can see a shot of the needles in their cylinder.
The diameter of cylinder is fixed. In my case, and in most, the cylinder is 4.5" in diameter. My machines came with a 54 needle cylinder and a 72 needle cylinder. The US Auto Knitter had 60 as the standard. I also have an 84 and a 100 needle cylinder.
To knit different sizes of sock you can either remove needles and do mock rib knitting, or change cylinders to one with more or fewer needles, or you can change the length of stitch by adjusting the tension on the machine.
You see in the picture of the needles that each needle has a foot.
And here is a picture of the cams on the inside of the machine, shown with the cylinder removed.
The feet of the needles sit on the ledge that you see in mid housing. When you turn the crank, this housing moves around the cylinder (which doesn't turn). When the cams (top of the picture) pass over the feet of the needles they force the needles to rise above their normal position, and then fall below their normal position, and finally, return to normal position.
When the needle rises, it goes above the yarn. On its way back down, it catches the yarn, and when it goes below, the ledge of the housing makes the latch flip up, which slips the loop over and makes the stitch. (Think "corking".) Weights are always hung on the work to prevent it from riding up on the needles and slipping stitches.
By turning the knob you see at the very top of the picture, it raises or lowers the set height of the cam. This causes the needles to be able to rise more high or less high. This increases or decreases the amount of yarn that will be taken into the stitch, which in turn makes the stitch longer or shorter in length. This is your tension.
Each yarn, of course, will have an ideal tension, but you can adjust up or down somewhat to get an optimal feel or sizing.
I find it difficult to explain things when you can't see my gesticulating, but I hope this explains some of the rudiments of making socks "on the round".