Hey Alessandro,
Please try updating to the latest version of X and Cornerstone and see if it works. I’ve just tested this in the latest version and the image size works. See https://youtu.be/03MuunGnF_8
Just note though that though it works, X does not support Visual Composer’s native elements. Please see how Visual Composer should be used within X at https://theme.co/apex/forum/t/extension-visual-composer/76
That means if something native in Visual Composer breaks in our theme, the furthest we could do as support staff is to post this to our issue tracker to let our development team know of the case. They will be the one who will decide if they will support the native Visual Composer feature.
If you want to take a look at the integration function, it’s in /x/framework/functions/plugins/visual-composer.php. Just note that we will not support any modification to it.
Thank you for understanding.