I have used US crochet terminology
When using the recommended materials, the diameter of the flower is 8 cm.

In this Poinsettia pattern you can find information on how to make a flower, a brooch, a keychain and a ring for a napkin
Also the pattern shows examples of room decoration with crocheted flower.

Materials used:
1 YarnArt BEGONIA (100% cotton, 50 g/169 m) red (№ 6328), dark red (№ 5020) and
green (№ 6332) colors. Yarn Weight is 2 - Fine.
You can use any other yarn you have.
The pattern uses two shades of red, but you can get away with just one.
2 Yellow yarn or beads. Any will do.
3 Felt base (diameter - 5 cm) or a piece of dense fabric.
4 Wire for beads, diameter - 0.3 mm.
5 PVA glue (optional).
6 Crochet hook № 2.5, needle, scissors.