What in this photo do you already know the size of?
In this sample, the left slab by the garage door is 9 ft wide and 8 ft deep. Choose A rectangle, tap the numbered targets, then enter A = 9 ft and B = 8 ft.
Scale set
Check the grid on the photo: each square is 1 ft, and the squares should lie flat on the surface. If they look skewed, drag a corner to adjust it.