My wife saw some ergonomically designed crochet hooks and decided she would like to try one. So she showed me a picture of one. I have made 6 or 7 of the long thin ones and this looked much easier because it had strength in the bulkier handle part. She said it is much easier on her hands.
The dia of the small end for the one she wanted was 5MM. It came out pretty good and she liked it so much she wants 3 more sizes.
Sorry about the picture quality, but my iPhone was all I could use at the time