Feed two eye's worth of food
The shape of the fish tells you everything about it's insides. Of course someone who is not used to looking at fish innards might not see the same things, but a little common sense goes a long way. The first thing to consider is the length from it's mouth to it's anus. More distance = longer digestive track. Also, look at the width of the fish. The rounder the belly the larger the digestive system. Mouth size is another determining factor - larger mouths swallow larger foods, which means they have to have the stomach to handle it. SO, as soltarianknight said, it's different for each fish.
A great way to determine if you are overfeeding is to add MTS to your tank, since their population will explode from chronic overfeeding. Certainly the inconvenience of dealing with an over population of snails is worth knowing to what degree you are overfeeding. They will also let you know if you are underfeeding, which is a problem that is never addressed. As fish grow, they require more and more food. However, we get accustomed to feeding the same amount of food and can't always tell to what degree the fish have grown over time. If the snails become far and few between, then that means you are not feeding enough. There are countless other benefits to having the snails in the tank (
sand, gravel or bare bottom), as well.
Honestly, I suspect that this eye/stomach thing stems from keeping
bettas. Otherwise, are we expected to look and add up the size of all the eyes of all the fish of a community tank?? How abstract. But when you are feeding a single fish, "how much" is much more important, and the size of it's eye is something that can be fairly accurately estimated.
