You can use another size that gives you the gauge you want.
7 is the size of the hook, usually it's in millimeters. The hook sizes also go by letters.
A 7 mm hook is a US size K or 10 and 1/2. The old British size was 2.
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
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
7 mm
The size of a small ball bearing.
7 mm
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.