I eat 1 serving, about 3 oz. of tuna. It is plenty for a salad or sandwich for me. It wasn't what I was eating that made me fat - it was how much. I found I can be satisfied with a lot less food than I thought by only eating 1 portion and then having more vegetables if I am still hungry.
I eat the entire can and count the calories. When I eat it it's mostly for the protein. I put dill relish in it and put it on 85 grams of romaine lettuce. 255 calories and sustains me for 2-3 hours. I have eaten only one serving until I realized the amount of protein. My body responds well to protein.
I don't eat the whole can but no matter what I eat, I count the calories, so that applies if I eat all of it. Good luck on trying out calorie counting! I love it.
I usually make a tuna/egg salad (with 2 cans) for a sandwich and split it up for 4 servings. So while i eat a serving of tuna, i also have an egg and some veggies mixed in