iAd supports different banner sizes for portrait and landscape applications. The exact size of advertisements depends on the device the banner is being shown on.
On an iPhone, a portrait advertisement is 320 x 50 points and 480 x 32 points for a landscape advertisement.
On an iPad, a portrait advertisement is 768 x 66 points and 1024 x 66 points for a landscape advertisement.
Please refer the following photo for details.