Whatever graphics application I'm using, I'll use that to scale the image to the size it needs to be. There's no standard "hand" size. It's just whatever size is needed for that client's hand. You'll probably want to look at the documentation for whatever application you're using to create the work to figure out how to print the page exactly as you have it configured in the image scaling.
What i usually do, to print my design for the stencil, is i get a rough drawing on the client, and trace a rectangle in which it fits.
then i take a ruler and measure both sides of the rectangle.
I can now print my stencil the exact size needed by resizing in my software. (Photoshop or Illustrator)