Go in Admin panel > Appearance > Editor and open functions.php. There you will see the default settings for the header, so change the ‘height’ from 200 to the value you want (in pixels) and save the file. That’s all.
I think the theme should accommodate whatever image I use for the header automatically though – users shouldn’t have to mess with php files, especially when the author has included some header customization options. I imagine I will have to manually readjust the height each time there’s a theme update?