Here's some info on feeding turtles. It gives guidelines on quantities.
http://www.redearslider.com/index_nutrition.html
You can feed as much red or green leaf lettuce as your turtle wants - unless he eats so much it gives him diarrhea. Then you'll want to cut back.

This doesn't happen to most turtles.
On other veggies and berries, you can give them a couple of servings a week. The head rule only applies to pellets. On veggies, a headfull might be about right for some things - like pieces of bell pepper. When I feed Spot blueberries, he will only eat three or four, so I don't give him a headfull. Same with tomatoes. For sweet potato, I think he'd try to eat a whole potato given the opportunity, but since it's high in sugar, I only give him a piece about half the size of his head.
Why don't wild turtles overeat? There are probably a lot of reasons - first of which is they don't get the opportunity. They have to hunt their own food - no one gives them nice, nuitritious, high protein pellets every day - and there just isn't that much to be had. Generally speaking, wild turtles eat more vegetable matter and less protein than captive turtles simply because the protein isn't as available. Also, wild turtles probably get more exercise and my guess is our captive turtles are more likely to get bored which, as we humans know all too well, leads to overeating.