When I was in Canada, I took one lesson in hand-spinning with a drop spindle. I found my spindle and bags of 'Top' when I was unpacking my boxes last week. Happily reunited with my tools and with a sudden desperate desire to make something right now, I sat down to make some thread.
My previous attempts at making yarn had been quite thick, in the realm of 'bulky' or 'chunky' weight. I had only made yarn usable with a 8-10mm hook/needle. So this time I tried to make my thread as thin as possible. I had trouble keeping the thread thin without breaking while I was spinning. In fact, the thread broke about 5 times while I was spinning, but I managed to spin it back together.
Finally I made a long length of thread and plied it together to make a 2 ply yarn. I'm happy to report that this yarn is definitely in the 'sport' or 'light worsted' range. I can use it with a 3.5mm crochet hook quite easily. So I'm very pleased with myself. A list of standard yarn weights and all sorts of useful things can be found on the Craft Yarn Council's Website.