When working in a smallish round, like a hat, you can sometimes use a very short circular needle.
But you might find that, when using one short circular needle, it can be difficult to join after casting on because the cast on edge has more tension in it than a knitted round.
Try this: Work the first 2 rows flat, and then join for working in the round. You can sew up the gap at the edge during finishing. Remember that you'll have to reverse the stitch instructions in the second row, treating it like a wrong side instead of right side.
(Image from Idiot's Guide to Knitting)
