Here's how we do it.
I'm 6'2" and have pretty average sized "man-hands". My index finger, when straight is 3.5" long from the tip to the middle of the third knuckle. With my finger bent like in the photo, I can get anything from 2.5-3". 3" is close. Most likely even a bit smaller, he looks to have stubby fingers, and I have no idea how tall he is.
If I hold a ruler up to his finger on the screen, I get 5/8". So, I divide 3" by (5/8) to get the ratio, which is 4.8. The fish, with the mouth extended (.5-1") even, is 3.75" long.
3.75" X the 4.8 ratio give 18", at the VERY largest.
If I use 2.5" the ratio drops to 4. And if I remove the extention of the jaws I get 3.5".
3.5" X the 4 ratio give a fish of 14" at the VERY least.
Average the two, 16".
I like math.