I can't speak to whether or not it's photoshopped, but I can tell you that fishbase lists 44 inches as the longest recorded carp, but it takes a long time for fish to reach these sizes. A more typical carp or koi will reach around 12 inches. A good sized pond koi will be about 24 inches. Some of the prize koi even get around 30 inches long, but this is with champion feed and pristine water.
EDIT: Here is a picture of a 26" koi being sold online for a few thousand dollars. But she's a beautiful fish. Click on the picture to see more info including a video of this monster moving.