NervousDave - patterns usually say what length. The measurement is from needle tip to needle tip though, not just the length of the cable.
With projects where you decrease a lot, you might need two different lengths. Or with something that decreases down to a very small number of stitches, like at the crown of a hat, you usually have to finish off on a set of double pointed needles. I use dpn a lot because I make socks, so I'm used to them.
Circular needles are great when you're making anything tube-like, especially in stocking stitch, because you get to do all knits and no purls because you never have to turn roubd and work on the opposite side. They also mean that you don't have to sew up the sides e.g. on a jumper.
You can also use them for non-circular knitting. Apparently some people use them all the time instead of straight needles,because they are lighter to hold. To use them for straight knitting, you don't join the 2 ends of the work for working in the round, you just turn over and knit the other way like on straight needles.