Lightbox / Image Size

well first of all there are a lot of snippets for that you can insert in your functions.php of your child-theme
i sometimes use this – and it depends on what i like to have bigger ( than the fullsize image is taken instead of the large one)

function change_lightbox_size() {
    return "full";
}
add_filter('avf_avia_builder_helper_lightbox_size','change_lightbox_size', 10);

function enfold_customization_change_popup_size( $size ) {
  return 'fullsize';
}
add_filter( 'avf_avia_builder_masonry_lightbox_img_size', 'enfold_customization_change_popup_size' );

to style the dimension of the image you can play with:

img.mfp-img {
    height: 70vh !important;
}