Cut your sashings the same length as your blocks and as wide as you want them although for a 9 1/4" block I wouldn't go wider than 2 1/2" but that's up to you. Your cornerstones should be a square of whatever width you choose. ie: sashing 9 1/4 x 2 1/2, cornerstones 2 1/2" square.
Hope this helps
I probably wouldn't even go that wide. However I might use a 4 patch as a corner stone and use 2 sashings if I wanted to make the quilt bigger.