Corner triangle stencil sizing
 

If a corner stencil says it is for 10" is that for the size square it was cut from?

i can't figure out what size to order. My corner triangles are from a 9 1/2 square cut in half diagonally

I would say that it's actually 10", so 10" is the finished size (square started out at 10.5"). To be sure, I would contact the company and ask.

Yes it is best to call the company. I know that one company marks their triangle/corner
Stencils by the size of the block they fit and not the size of the stencil.