Originally Posted by MTS

Yep, your two squares would be 35 inches.
This is for blue-ish fabric in the corners.

Look how big your quilt is - 91" square.

So you can see how you'd need to start with a really big square to get triangles to fit in each corner.

I would suggest you really starch the squares BEFORE you cut them, as you will end up with a really, really long bias edge.

Sometimes I make the squares oversized, draw the diagonal line, and then stay stitch on each side to stabilize it BEFORE I cut on the line.

You want to make sure that you handle the pieces gently so they don't stretch out of shape.