6×9 car audio speakers and mid-range speakers are the same thing. These speakers can provide you with the mid-range of sound, which is the most natural sound to most people’s ears. When you are installing these in your vehicle, they are usually installed in the side door panels of the vehicle, which tends to give you a surround sound effect. The bass or subwoofer is usually installed furthest away from you in the rear of the vehicle, while the little tweeters get put in the front panel. The size refers to the size of the actual speakers themselves. Those oval ones can be 6x9s speakers.
For more info, see Tips for Finding the Best 6×9 Car Speakers