Pattern is written using U.S. Terminology.
It is better to crochet from yarn that holds shape well in the finished product. From soft yarn, the brooch will also be soft. Starching or similar processing may be required.

I crocheted Dacron thread, 70.5 g per 1000 m of yarn, hook 0.55 mm. The size of my brooch is 2 3/4” (6.5 cm) in diameter. The brooch holds its shape perfectly.
And I crocheted cotton thread, hook 1.15 mm. The size is 4 3/4” for the first option and 3 3/4” for the second option.