I'll try to help...I'm a pretty new knitter myself, but I'll answer what I can.
"What sizes to they come in?"
Do you mean size or length?
If you mean needle size, circulars come in all sizes from 0 up.
If you mean length, I beleive they come in 16", 24", 32" and 40".
I'm probably missing a few, but those are the lengths that seem to be the most popular.
"What would you use for little tubes like socks?"
For teeny things, you would use eiter Double Pointed Needles (DPN's), two circulars, or what I use, Magic Loop.
"What would you use for flat knitting?"
It really depends on what you're knitting and how big it is.
I recently made a baby blanket, and I knit it flat on a 24" circular.
I could have used straight needles, but I found the circulars so much easier on my arms.
You don't have to hold up as much weight. With circs, it all just rests in your lap.
But if you're knitting something really large, like an afghan, you could use a longer curcular, like a 40"
If knitting something smaller, like a baby sweater, you can use a shorter circular, like a 16".
I recently ordered a set of Knit Picks Options Interchangable Needles, with the input of the wonderful members here.
You can google them to see exactly what they are but basically, it's a set of circular, interchangable needles.
You get the tips from size 5 to size 13 (I think?), and you get 4 different length cables that connect to the tips.
It's much less expensive than buying each size and length seperately, you know?
I hope I answered your questions.