I've spent some time checking all this out with the same photo resized in steps from 1920/1680,1320/1024/800/640/320 landscape widths and find that both the post display as well as the Lightbox remain identical right down and including 900 x 600. Lower than that, they get smaller.
It is quicker to just click on the post photo to activate Lightbox than to go find it in the post attachments The result is the same.
Here are 5 versions of the same photo
1320 x 880
1024 x 680
900 x 600
800 x 400
640 x 480
While your post screen is wider than mine the overall effect is the same.
So, does this mean 900 x 600 is the real true max.? In practice it certainly seems that way.
The reason I dwell on photos is that our forum is a sport/hobby based one and photos are it's lifeblood.