The green box is the default text box - if you want to change this click on the green line, you will see the adjustment blobs. You can then type in the width and height on the info bar.

From your original post I suspect you should not be changing the page size as you want 4 coupons on each page, just set the text box to be the correct size and repeat 4 times. You might also need to make sure the units are set to inches if this is what you are working in.