If you can get your hands on a simple projector, like the ones they use in school, you can project any picture on the wall in any size you want and then simply trace the projection, or paint right over it.
Now that schools out, you may be able to borrow one from a local school. It couldn't hurt to ask.
Good Luck!
Edit: Here's an example... Video
Now that schools out, you may be able to borrow one from a local school. It couldn't hurt to ask.
Good Luck!
Edit: Here's an example... Video