easy slider min height rule

hi

if you go to this page http://www.codecreative.uk/landing-page-web-a/

im really happy with how it looks but has a minor issue with the really easy full width slider. Height reduces as screen width reduces. I want to enforce a min height of 400px to ensure no text gets cropped on smaller resolution screens. Is this possible please?

I added a class name to the slider called landingpage and I used this css but it fails
.landingpage {min-height: 400px!important;}