Judging objectively how much is a good portion is really difficult. Personally it has been one of the hardest lessons I've had to learn. Before I started I used to eat fairly healthfully but
huge portions.
What helped me was keeping a food journal and calorie counting. If I don't count calories I start slipping back into my old habits (specially as I have a husband who is 6'4" and eats like a horse

).
I used to use pen and paper and visit a nutritionist for this but nowadays there are heaps of online calorie counters and food journals that make this super easy. Let me know if you want to know which one I use.