Hello there,

Unfortunately there is no built in functionality to preview only featured image Full Width. You can only set a whole blog post to be in Full Width using this Custom Css code

body.single-post .eltdf-content .eltdf-content-inner>.eltdf-container>.eltdf-container-inner{
    width: 100%
}
body.single-post .eltdf-blog-holder article .eltdf-post-image{
    width: 100%;
}
body.single-post .eltdf-blog-holder article .eltdf-post-image img{
    width: 100%;
}

However, this will stretch whole content to be 100% wide, not only Featured Image.

This answers the second question as well, using this code your blog post will be 100% wide and so the images added to the post will be 100% wide.

This Custom Css code needs to be pasted into Elated Options > General > Custom Css field https://www.screencast.com/t/xn2yt5ur4 .

Please let us know if this solved the issue and if there is anything else we can do for you.

Thanks.