Portion size does matter, but only in that most people eat portions that are far, far too large for their daily caloric intake requirement. In other words, for many people, their overly large portions are usually providing them with far more calories than they burn in a day (basal metablolism + activity). So for a lot of people, simply reducing portion size to a more reasonable level will result in weight loss without any other changes. Of course, dietary changes are also a good thing for a number of reasons, and also help with overall health, as does exercise (which serves to raise the amount of calories you burn, not to mention promote overall health in many ways).
So your portion size and dietary intake need to be matched with what you're trying to accomplish and how much activity/exercise you're involved with each day. Someone lifting weights and attempting to bulk up is going to have a very different diet and caloric intake than someone just looking to tone up and get in better shape. A sedentary person that does little but sit at a desk all day and then go home and flop on the couch is going to need far less caloric intake than someone working construction, or someone that comes home from a desk job and works out hard or plays multiple times per week.
As for Omega-6 fatty acids, it's true that the average American diet contains far more than you need. Fish oil supplements that contain just Omega-3, as mentioned, are the way to go (they're easy to find). Specifically, you would want something that provides good amounts of EPA and DHA, as those are what really matters. Omega-3 supplements have a lot of probable benefits - in my view, it's one of the very few supplements that are actually worth taking every day for the average, reasonably healthy person. Check with your doctor first, though, if you take any prescription meds - especially anything that thins your blood or promotes bleeding.
Honestly, though, excercise is the key. Exercise, exercise, exercise. That, more than anything else, will help lower your cholesterol and improve your overall health. I would rather be a person that eats junk food all the time but exercises hard each day than a person that doens't exercise but eats "healthy." A balance of the two is ideal, but for most people, exercise is needed most. And yes, walking 30 minutes a day will help if you're sedentary, and it's better than nothing, but I'm talking about real exercise.