Currently Admob supports the following ad formats, these are also defines as constants in the GADBannerView.h

// iPhone and iPod Touch ad size.
#define GAD_SIZE_320x50     CGSizeMake(320, 50)

// Medium Rectangle size for the iPad (especially in a UISplitView's left pane).
#define GAD_SIZE_300x250    CGSizeMake(300, 250)

// Full Banner size for the iPad (especially in a UIPopoverController or in
// UIModalPresentationFormSheet).
#define GAD_SIZE_468x60     CGSizeMake(468, 60)

// Leaderboard size for the iPad.
#define GAD_SIZE_728x90     CGSizeMake(728, 90)