Answer: ~ 0.68
See picture below:
Green and red lines are =1
Side of the square=x
Sides of yellow triangle ABC: AB=1, BC=x/2, AC=x+(1-x)/2=(1+x)/2
Since AB^2=BC^2+AC^2, we have 1^2=(x/2)^+((1+x)/2)^2 => x^2+1+2x+x^2=4 => 2x^2+2x-3=0 => x=1/2*(sqrt7-1) ~ 0.823
Area of square = 0.823^2 ~0.68