Adding header pixel dimensions to the Custom Header Page

Hi, I need a little help.
I'm trying to add the header dimension parameters to the code so it shows up in the custom header area when someone tries to change their header banner.
Where it says "You can upload a custom header image to be shown at the top of your site", I want to add the pixel dimensions for the image so they know the maximum width & height their image can be.
Where in the code do I place it so it shows up on the page? The url directory tells me to go to the themes.php file but I don't know where to place it.

Help anyone?

Thanks,

Jafar