I doubt anyone is going to measure the ears of the Mickey Head on your t-shirts.
I've seen a lot of posts here on the Dis where I think people are getting a wee bit obsessed with making their trip "perfect" and end up way over thinking things. I think you win!
Seriously though, nobody in your family is going to care if the head to ear ratio is exactly right, they are going to have fun even if it is way lopsided. I promise.