I have no reference of size from another object in the photo and it's a close up, so it makes it hard to gauge how big it actually is. You can make lots of things look bigger from different angles of photos. I personally would've thought it was about 18" by looking at the pic (I had a 20"), but I would've never guessed it was a 40" fish. But I'm a hobbyist, not an expert.
Bottom line, is this a fish you're selling or were you just posting to show the potential size and development of the smaller ones you have for sale?