"Take two quart size zip lock bags and see if you can squeeze one day's worth of food into them."
That is a good place to start, but don't stop there. IME, with practice you should be able to fit a day's worth of food, up to 22 oz of carefully chosen, carefully compressed food, in a quart zip loc baggy. Another, more efficient method is to pack things like soup, oatmeal, trail mix, etc in one large bag and measure out daily rations from it. Larger bags not stuffed to the max like a quart baggy offer more opportunity to make them conform to available space. The downside of this method is that it is not as neat and tidy as one quart zip loc per day, but you will be able to get more food in your Weekender.