I feed the NLS Betta pellets. They are 1mm but tend to be on the large end of the range. I feed an average of 4 pellets, twice a day. I only give him one at a time. After he eats one, he does a lap around the tank. After three, it takes him longer to return to his feeding spot. After the fourth, he is completely uninterested in food. I see that most people here say the typical betta will eat until he pops; I guess he isn't typical. If a pellet sinks he will ignore it until I siphon the gravel. That's why I've always fed him one at a time. When I look at him from above, he has a nice gentle taper to his body, and he doesn't look like the photos of underfed bettas that I've seen.
I noticed that the amount he eats is related to his water temperature. When I first upgraded to his 10 gallon tank, my 25 watt Hydor didn't get the water up to 80. It was closer to 76, and he only ate about half of his normal intake. As soon as the new heater got him back to 79, his appetite was back to normal. This week I've raised the temp a little to assist the Dr. Tim's that I used, and he is eating a little faster than normal, even skipping his first two laps around the tank. Higher temp --> higher metabolism --> more food. He's never been bloated, and since I feed him the good stuff, I don't worry about overfeeding.