The length is 16cm

Sushi, if you notice the diagram carefully, you will notice that the length of A is equal to the total widths/breadths of B and C.

But the thing is, the three rectangles are...identical. This goes to show that the width of B must be the same as the width of C (and of course A as well).

So, the length of each rectangle must be two times the width of each rectangle.

Surrounding the outer perimeter of the large shape are a total of three lengths and four widths.

If 1 width = 1 unit...
...then 1 length = 2 units.

So, 4 widths = 4 units...
...and 3 lengths = 6 units...
...so the total perimeter of the rectangle is 10 units.

10 units ==> 80 cm
1 unit ==> 8 cm
2 units ==> 16 cm

So, the rectangles all have lengths of 16 cm each.

Rectangle A has a length of 16 cm.

Thank you Eric for your detailed explanation! Sincerely appreciated!