With responsive headers, it is NOT the size of the image that matter most, it is the SHAPE. If you just simply decrease the image size, you just end up with poor quality images. If you change the shape of the image, you can control the height of the plugin. If the image is spending full screen, you need to re-crop the image outside of Weebly to a more rectangular shape.
RECTANGULAR– This will display smaller
SQUARE– This will display almost full screen