I want product gallery images to be full size when lighbox is open

Are you referring to a height and width set on the image with the style attribute (css)? Or is the image itself a cropped image with a height and width appended to the image url?

If the first, then the style attributes are there to ensure the lightbox will fit in the current browser window.

talking about image with style att.
it looks like this
<img src=”bla.jpg” style=”height=x; width=x;”/>

Correct. Those style attributes are added so the image and lightbox fit inside of your browser window. Otherwise massive images would make it impossible to close the lightbox.

Cant i set what will be max size?

It’s all handled by the lightbox script. If you don’t want images to be resized, don’t use the lightbox.