how to change the pixels of an image in a lightbox？

or – you determine a different image-size as source for the lightbox image
for masonry galleries – this to child-theme functions.php:

function avia_change_masonry_thumbnail_link($size){
  return "full";
}
add_filter('avf_avia_builder_masonry_lightbox_img_size', 'avia_change_masonry_thumbnail_link', 10, 1);

then it will take the full image size ( original uploaded Image ) for it

btw: for different images ( image lightbox or gallery etc.) there are different filters to use
avf_avia_builder_gallery_image_link or
avf_avia_builder_helper_lightbox_size

PPS: for your mobile rendering of the image : no solution yet.