Header Image won't let me change size

On the Free version, I was able to use a size slightly wider than the recommended 300x125, however on this "PRO" version, I have no ability to change the size! From the Customizing Section: While you can crop images to your liking after clicking Add new image, your theme recommends a header size of 300 × 125 pixels. However, that size is set and I am unable to set my image to the size that works better for my site.