Scale width and height by desired square meters or foot

Hi there.
I need to draw a rectangle of 9 square meters. I started with 3x3 square, but now I want to scale the width and I need the height to be scaled automaticlly in order to get a 9 square meters.
It is just a simple task but I havent found a solution. Thanks.

This idea is already discussed:

This may help. Thank you!