The area of a shape is the amount of space the shape can occupy.
The dimension that maximizes the design is 20 inches by 20 inches
From the question, we have:

Let the length and width of the graphic (without the margin) be l and w.
So, the dimension of the whole graphic is:


The area is then calculated as:


Substitute 

Make l + 4, the subject

Subtract 4 from both sides

The area of the design is:

So, we have:


Plot the graph of 
From the graph (see attachment), the maximum is at:

Substitute
in 

Divide both sides by 20

Hence, the dimension that maximizes the design (<em>excluding the margin</em>) is 20 inches by 20 inches
Read more about maximizing areas at:
brainly.com/question/3672366