Easy and fast project.
Works in the round, no slip stitch to join every round.
This is a mini bag pattern.
To increase the size, increase the no. of the base chain.
Here's the math
Chain 8 will result to
(with 3 sc on both corner)
R1 - 16 sts (3+5+5+3)
R2- 22 sts (6+5+5+6)
R3 - 28 sts (9+5+5+9)
Both corners will increase by 3 sts per round.
3+3 = 6+3 = 9 and so on.

If you want to increase the size of the bag, just add more chains to the base chain. But the rule in the corner remains the same.

Stay Creative.