I take a picture and crop it down to the exact width of my tank. My tank is 48" wide, so I resize the picture to 480 pixels which works out to 10 pixels per inch.
I then copy the portion of the pic that contains a good side shot of one of my fish into a new image. If that image is 40 pixels wide, then I know the fish in question is about 4" long. It doesn't give me an exact measure, but it only takes a minute or two and gives me a decent estimate.