Set image dimension for WP_Customize_Image_Control images

I managed that with css. This is how i did it:

1. On your header.php file add a class to your image tag like this:

   img class="logo" src="" alt=""

2. Then, on your style.css add the logo class:

   .logo { max-width: 200px; }

If needs any change afterwards just go to the Editor and change the value directly in the style.css file.