To figure the size of the square used to cut a QST, you add 1.25" to the finished size of the block you want to create. If your block's finished size is 9.75", you would need a square that is 11". I assume that your 9.75" block is unfinished, so finished it would be 9.25", and adding 1.25" you get 10.5", which is what the pattern calls for.
You can look at this link for the math -
http://www.twthreads.com/pdf_files/HSTmath.pdf