To span 16 feet you need to create a proper header. In your drawing, the bottom picture is close to what you have to do.
Not knowing your snow load or the engineering it takes to span 16 feet, I would look into how every two car garage is built in your area. Code is probably an engineered beam. Here in the South, and also out West, we used two 2x12's with half inch plywood between them, glued and screwed together. I do this in place because it's so heavy. The post should be notched to rest the beam on top of solid wood. More then likely, you will need three 2x12's to make the span with a snow load.
Before building a proper beam, compare the cost of an engineered beam, or glulam beam. They run about a hundred bucks each, and in the grand scheme of things, you are not saving very much by building your own header, especially if you need to triple up the 2x12's.