Can make it any size.
Crochet design is a Wave design, for a rectangle pillow/cushion..
You will also need (if you require) a backing fabric, same size as the pillow plus seam allowance.
A zip 40 cms/ 16 inches long (optional)
Pillow/Cushion pad 45 cms x 65 cms / 17 inches x 25 inches.
You will need 5 x 100 grm skeins/balls of yarn - i of each chosen colour.

Errata - foundation chain 83 not 84 chains.
Row 1 - (RS ) With yarn A, 3 chain (counts as 1 treble) 1 treble in next stitch. Should read, 2 treble in next stitch, not 1 treble