Answer
12 inches by 12 inches
Work Step by Step
$(x-4)(x-4)*2=128$
$x*x+x*(-4)+x*(-4)+(-4)(-4)=64$
$x^2-8x+16=64$
$x^2-8x-48=0$
$(x-12)(x+4)=0$
$x-12=0$
$x=12$
$x+4=0$
$x=-4$ (we are talking about dimensions, so this answer is not valid)
$x=12$
$(x-4)(x-4)*2=128$
$(12-4)(12-4)*2=128$
$8*8*2=128$
$64*2=128$ (true)
