Resize big images
 

Is there no way to Scan and resize big images?

No because how would one do that?

The main image is the one that is used to build the medium and thumbnail images.

There is no reason to rebuild the main image.

If you would like to have full control of lightbox and popup window outcome

Max allowable image width in pixels is set to 3000 in my gallery and I now would like to make all the photos smaller for better viewing experience

Yeah there is no such setting. Large images are shown in a popup window.