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")...
If you're getting the Elongatus from a store I recommend you just do a swap with your Rhom and try to see if they'll give you a few dollars store credit to balance the trade. That way you'll have the new fish that you want and won't have to deal with the hassle of waiting for someone to buy...