You can consider that each little square has the dimenions of 1cm x 1cm.
The biger square has a side with four little squares, then, you can conclude that the sides of the biger square is 4cm x 4cm
side length side length
4 4
You can observe that there are 16 litle squares inside the biger one. Then, the are of the biger square is 16 cm²
Finally, you have :
√16 = 4
