It's easy to make a rectangular granny, the secret is in the second row!
I am, of course, assuming that you already know how to make grannies....
And I'm also assuming that you can kinda see what I'm talking about.... (!)
On the second row....
You add an extra 3 trebles in the middle of the middle cluster (rather than just the corners).
Of course, only doing this on 2 opposite sides and voila, a rectangle.
And then just make it as big as you want (without having to increase again!).
does this make sense??