This is the mural I just finished in my son's room. I painted a 3 x 4 foot rectangle using magnetic primer. I then top coated it with house paint and painted a copy of one of Evan's dragon drawings on top. You can see in the photo that he already has magnetic words up on it to play with.
I found the magnetic primer to be very sloppy to work with and did 6 coats of it because the instructions indicated the more coats, the more magnetic it would be. I did not find much difference between 3 coats and 6. It does the job though.