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
A 7mm hook refers to a crochet or knitting hook with a diameter of 7 millimeters. It is often used with medium to bulky weight yarns, allowing for the creation of various projects such as blankets, scarves, and hats. The size provides a good balance between creating tighter stitches and maintaining flexibility in the fabric. This size is commonly used by both beginners and experienced crafters for a range of patterns.
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
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.
22 mm or 7/8" will work