I think the Newegg one is correct. I have never shopped from there because I am in the UK, so I can't be sure though.
Amazon, I think, measure the whole box. They sell almost anything you can imagine, so they understandably just measure the whole thing and put it up for sale. Newegg is dedicated to PC components so they might measure the card itself, knowing that is what people need to know.
I have an MSI GTX 760 though, and it's great! Obviously it is a large card, but no longer than the average high end, full size sort of card. It should fit in your case if it's a mid or full tower, but be careful.