Hmmm... that depends on if you are talking about knitting a flat piece on circular needles or an actual circular piece (not turning at the end of rows). I have circulars that are only 6" long, which would make them the right size for making socks, but it hurts to use them. The tips are too short to hold comfortably and the flat cable is too rigid, causing lots of tension in my hands. So, for socks I either use 2 circulars (doesn't matter what size) or one long circular (somewhere between 32" and 60"). My favorite is 60". The cable on KnitPicks needles is soft enough that I don't fight the extra length and it's long enough for anything I've made.

But if you are looking for just one circular that "fits" the item you're making, 16" is just right for a large child's hat or an adult hat. 24" would be sufficient for a sweater knitted in the round. Usually, if a pattern calls for circulars, it also suggests what length would work best.

I have some circulars well over 20 inches long that I bought on Ebay. I think they might work for an afghan but I have never used them. If you are making something circular such as a baby hat, you don't want more than 16" tip to tip. The 24" range is good for a sweater, as Donna said. A circular needle set that lets you change the tips on the cables works well because it gives you all sorts of flexibility. And then we all have our favorite brands of sets.