Which board will Mia use for the length of the shelter, given that 1 foot = 12 inches? What is the width of the shelter, measured in feet?

Mia will use a board that is 144 inches long because 12 x 12 = <<12*12=144>>144

The width of the shelter is 12 feet because 144 / 12 = <<144/12=12>>12. Answer: \boxed{12}.