Replace meta slider in home with image 1920px width

Hello,
I’m trying to change the slides in the homepage with new ones 1920×550. I tried different combinations but always with te same result: my images are cropped at 340px.
I set the slider’s settings to 1920px width and to 550px height, using Flex Slider and theme default.
I also insered this code found in other thread but nothing appens.

/* Vantage Theme, remove max width/layer centering.
Usage: Copy and paste this code into Appearance > Custom CSS or your own Custom CSS module.
*/
body.responsive.layout-full #page-wrapper .metaslider .full-container,
.layout-full #page-wrapper .metaslider .full-container {
max-width: none;
}

Unfortunately I’m forced to work in local bacause of an hosting problem.