Do you have any good resources for diet things? I'm interested in this.
It took a few months before I figured out what my portions were, since I don't feel full until about 15-20 minutes after I am. By the time I knew how much I needed to eat in one sitting, I'd become used to eating that amount, so it was a breeze.
I do eat whatever I want, with the exception of processed foods. If I can't make it at home with whole ingredients, I'll likely not eat it. I do cheat every so often, but not much, and not a large amount (I love a box of Raisinettes during a movie.)
As for not being able to think straight after lunch, I used to get that way when I'd eat too much. I used to love feeling full, but it would definitely affect brain function. Now, I get ill when I overeat. I also try to eat a high protein, high fiber breakfast, because it seems to get me further through the day than anything else (I eat a small late lunch).
I read plenty of literature on estimating the dietary recommended portion sizes in food, but nothing on how much food I personally should eat in a sitting.
As I say, it did take a few months. I also needed to eat a little slower - I was a fast eater of big meals. Mostly just experimentation and willpower - you have to get used to waiting to feel full. I go by the two fist rule: your stomach's about as big as your two fists put together.
My family also likes to go out to eat a lot. I ask for a box as soon as the food comes, and put half of the food in it. I used to finish the other half, but now I'll usually end up adding a bit to the to-go, and making two meals out of that later. If I'm by myself, I usually just order an appetizer (I do find myself eating more of just one type of dish per meal). If I go with my family, I'm ordering a meal, because, hey, free food!