4-6" is the average adult size. After 15 years, it is unlikely he is adding much extra length. The easiest way to measure them is to tap his nose so he closes up, then measure a straight line from the front of the shell to the back.
Now- as box turtles get older, they do need less food, so he could easily be getting fat. A healthy box turtle can close up completely and easily. If he cannot, he is overweight.
A serving size for most turtles is only about the size of their head (not head and neck, just head). Babies should eat daily, but by adulthood, most of us only feed them about every three days.
Note: There is really no such thing as 'healthy' pet store box turtle food. You can use it as part of a bigger diet, but boxies should be getting a good, varied diet. I use a 'One Third Mix' for mine.
The One Third Mix is this: Over the course of a week, I am going to feed my Box turtle the following:
- 1/3rd 'bugs'- worms and crickets mostly, but other bugs as possible.
- 1/3rd 'meat'- mostly well-soaked weight-control cat food pellets, but might include pinkies, cooked chicken, commerical box turtle chow, canned 'carnivore' diet, hard-boiled eggs (and shells), etc.
- 1/3rd 'veggies'- mostly dark leafy greens (turnip or dandelion greens, or escarole or endive lettuces), colorful veggies like squash, mushrooms (they LOVE mushrooms), berries, and a little fruit.
- Sprinkle every meal with a dash of calcium suppliment, and every other meal with a pinch of vitmain mix.
You can make your life easy by just feeding some bugs, wait two days, feed some meats, wait two days, feed some veggies, wait two days and repeat- but this is not a great way to provide a lot of variety.