For this, you can go with a graphics designer which will create banners in various sizes as per your need. One of the most performing banner size is 300 x 250, though if we go with recommendation, Google recommends that only half of the banner should be visible above the fold. Specifically, mobile video ads use that size. You can also go with popular ad sizes which include the 300 x 50 mobile ad banner. However, you need to pay attention to ad analytics to decide on the right mobile ad size for the audience you have targeted.