If one square is divided into 9 smaller equal squares, then they have to be arranged in 3 lines of 3, that is 3 smaller equal squares per side of the original big square. That said, the area of the big square is equal to the multiplication of 3 small squares sides times 3 small squares sides, call x the length of the small squares.
So,
area = 9 = 3x*3x
9x^2 = 9
x^2 = 1
x = 1
therefore the smaller squares have sides of 1 unit