How to hide the header and shorten the width of Mobile menu bar in WordPress

Are you facing trouble with the banner height and width in WordPress? Our technical experts are here to help you out.

1.My theme has already a big height banner which is unusual. Please suggest me a CSS so that the banner can be hidden or can be decreased in height as I want.
2.What is the CSS for decreasing vertical width of Mobile menu bar?
Please let me know at your earliest.