what size should the rectangle be?

Since it makes 36 cookies, sliced 1/3 inch thick, I'd roll each half out to a little over 6 inches wide. In the picture it looks like the thickness of the swirl is about the same as the thickness of the cookie itself, so maybe roll out to about 1/3 inch thick, regardless of length? Then roll up from the 6-inch side. I see another issue too: 16 oz of butter is not 2 sticks; 2 sticks is 8 oz, 16 tablespoons. I think you want 20.5 Tablespoons of butter total, 16 for the dough, 4.5 for the filling