My set is 8", which is fine for things like finishing off hats or sleeves. They're also 4-needle sets. The shorter ones I have are come with 5 needles, so I'm probably getting about the same circumference out of them. In which case for me, it comes down to whatever's on hand. I do find the shorter sets with more needles a little less fiddly when working with tiny numbers of stitches like at the finish of a hat crown, but for a sleeve, which would be finishing off with 30-50 stitches, longer ones are just fine.
You can do most of a sleeve with a small circular needle, but toward the end, you'd likely want the DPNs unless you're going to go with the magic loop method. Which I only use to do sleeves two-at-a-time, which is awkward when you're working sleeves top-down from the body.