Specific image size in Lightbox

Hello:
Thanks for the plugin.
I have very large images to use in a homepage slider.
I want to use them in a gallery too, but when they is showed in lightbox, there are very heavy to download.
It’s possible to set a medium or large image to be showed when click on a image showed in a lightbox?
At present, lightbox is showing the original full size image, and I need define a medium or large size to them.

Best regards,
Javier.