Lead Developer
The height is controlled using GP Premium in “Customize > Layout > Primary Navigation”.
However, altering the height will change the height of the entire navigation.
If you need to use an image, just replace the border-right attribute with your background-image.
Documentation: http://docs.generatepress.com/
Adding CSS: http://docs.generatepress.com/article/adding-css/
Ongoing Development: https://generatepress.com/ongoing-development