I would cut the squares for the sides 12-1/8 (12.125) inches
and the corner squares 6-1/8 (6.125) inches.
I think they might be a smidge (about 1/16 inch) too big.
The way I arrived at these numbers is this:
For the side triangles:
8.5 squared plus 8.5 squared = 144.5
The hypotenuse is the square root of 144.5 = (approximately) 12.02
For the corner triangles:
the square root of 1/2 of 8.5 squared = the short side of the corner piece
8.5 squared = 72.25
1/2 of 72.25 = 36.125
the square root of 36.125 = (approximately) 6.01