Hello @zabcia,
The height of the image in the featured slider in fabolous fluid theme is set by the height of the actual image you are using as the featured image for the slider page.
While we can provide you some custom CSS to force reduce the height of the current images, it would be best if you would upload the image of your desired dimension ( of desired height ) for the slider.
You can also try the CSS below in your Admin Dashboard > Appearance > Customize > Additional CSS :
#feature-slider .wp-post-image {
width: 100%;
height: 450px;
}
Also, for any issues further / future reference, you can also choose to post in your queries to the theme’s official support forum for dedicated and swift support at : https://catchthemes.com/support-forum/forum/fabulous-fluid-free/
Hope this Helps,
Best Regards !!