You have full freedom to use the size. The maximum size will be the container / content size. It is your decision how you will add the ads. If you use image and if it will be larger than the container in width, the image will be automatically adjusted with the container to 100%. If you use javascript ads like Google AdSense the ad has pre defined width. You can use HTML to set the position to left, center or right. Because the Theme is Responsive, the width is dependent on Device Screen. So, we can’t specify any dimension in pixel. It is you who will take the decision about the Ad Size. You can Visit This Site and This Site to see how they are managing Ads. You should go to their Single News Page, too.
