[Theme: Yoko] custom header size

In functions.php find on line 68 and 69 the specified default width and height settings. Change that to your image size. Bare in mind that cropping will occur, so your actual image needs to be a few pixels larger for it to crop to a hard edge etc.

The functions.php is what tells the theme options (under appearance, theme options) what the default size is. So if you change it within functions.php your theme options setting will reflect whatever size you specify. This means you are actually editing the function of your theme!