You can buy mouse gaurds here, which are the right size to NOT let a mouse in and are metal so they can't just chew their way in. You want an opening that is not more than 3/8" high (10 mm) and not less than 1/4" high (6 mm). This should keep the mice from getting in. How wide is up to you, but I would leave at least a couple of inches (5 cm) or so.
