Since this is an external plugin, we usually recommend contacting the developer/creator of the plugin since this is an external issue (outside of the theme itself).
If I were to take a quick guess it looks like you’ll probably need to change the #megaMenu height to “auto” since it shows at 57px. You could try adding this to your responsive section of your css. There are a few other areas that may need adjusting as well, but you’ll need to contact the developer of the plugin for further support on how to properly make this customization.