I don't think portion sizes matter, as long as you realize that you are not expected to eat everything on your plate all at once. I think problems happen when parents worry if their children don't eat all the food on their plates. I think that when children are conditioned to eat everything they see in front of them, regardless of whether they are physically hungry, they eventually lose the ability to sense true physical hunger.*
This never happened with me because I was always a fussy eater as a child; my parents were happy for me to eat anything at all, let alone finish my plate. I've always found that being encouraged to eat something when you're not hungry just makes the food more unappetizing.
I usually can't finish a large British or American meal. Anyway, if I eat past my "full point", I wind up getting sick, which means that in the end, I've digested less than I would have if I'd eaten less in the first place.
* In Psychology 1 in University, we learned that there are 4 things that make you want to eat:
1. Low blood sugar
2. Low level of nutrients in the cells
3. Empty stomach
4. Sensing food (e.g. seeing or smelling it).
Overweight people are more likely to respond to reason #4 than to the other reasons.
Note: cross-posted with MeShell