You can use another size that gives you the gauge you want.
A U.S. size 7 yarn hook is 4.5 mm. http://www.crochetmagazine.com/hook_sizes.php
It's about the size a a 12 year old pinky
First off, there are two "types" of needle sizing. The first is the US method of measuring needles, and the second is the UK/Canada method of measuring. Therefore, there are two answers to your question. For US knitting needles, there are sizes: 0 (2.0 mm) 1 (2.25 mm) 2 (2.75 mm) 3 (3.25 mm) 4 (3.5 mm) 5 (3.75 mm) 6 (4.0 mm) 7 (4.5 mm) 8 (5.0 mm) 9 (5.5 mm) 10 (6.0 mm) 10 1/2 (6.5 mm) 11 (8.0 mm) 13 (9.0 mm) 15 (10.0 mm) 17 (12.0 mm) 19 (16.0 mm) 35 (19.0 mm) 50 (25.0 mm) For UK/Canada sizes, there are: 14 (2.0 mm) 13 (2.25 mm) 12 (2.75 mm) 11 (3.0 mm) 10 (3.25 mm) 9 (3.75 mm) 8 (4.0 mm) 7 (4.5 mm) 6 (5.0 mm) 5 (5.5 mm) 4 (6.0 mm) 3 (6.5 mm) 2 (7.0 mm) 1 (7.5 mm) 0 (8.0 mm) 00 (9.0 mm) 000 (10.0 mm)
21 mm
7 mm
7 mm
The size of a small ball bearing.
Here you go: 1) Slip Stitch 2) Single Crochet 3) Half Double Crochet 4) Double Crochet 5) Treble Crochet 6) Single crochet increase 7) Single Crochet decrease 8) Double Crochet increase 9) Double crochet decrease 10) Treble Crochet increase
The epiglottis is a flap of tissue that is found at the entrance of the larynx. Its size is about 7-9 mm in length.
Here you go: 1) Slip Stitch 2) Single Crochet 3) Half Double Crochet 4) Double Crochet 5) Treble Crochet 6) Single crochet increase 7) Single Crochet decrease 8) Double Crochet increase 9) Double crochet decrease 10) Treble Crochet increase
22 mm or 7/8" will work
22 millimeters is 0.87 inch.