You want 13 inches as a minimum, or it ruins the driving experience in my book. Don't forget you'll also need a boss to fit your chosen wheel.
How about getting wood? They look better bigger.
I like this purely on the 'how about getting wood' comment - immature mind.
I dont really like the wooden look - i like modern inside but classic outside (As weird as that sounds!). What is the standard mii steering wheel size? :)