Enlarge it 20% and copy off onto thicker cardstock type paper. Copy paper will work in a pinch--it just won't last as long. You'll probably have to take it to a copy shop to print it out unless your printer will take ledger/tabloid size paper.
Then use an exacto knife or sew with un-threaded needle to make your holes for the chalk or marking pen. You've just created your own 12" stencil!