The way to capture a large sun in a picture is to use a long telephoto lens. The longer the focal length, the large the sun will be. To get a sun as large as you see here, I used a 500mm telephoto lens. Since such a large sun can adversely affect the meter reading, check the LCD monitor on the back of your camera to make sure you like the exposure. If it is too dark -- and this is typical of shooting into the sun -- then use the exposure compensation feature built into the camera to tweak the exposure in 1/3 f/stop increments. Alternatively, you can also re-compose the shot so the sun is placed further away from the center of the viewfinder. In that way, it has less influence on the meter results.
This picture was taken on Inle Lake in Burma.