I would take a look at Apple's sample code
for integrating iAD exactly how you describe, particularly in ContainerBanner/ContainerBanner/BannerViewController.m. What you are looking for is ADBannerView's - (CGSize)sizeThatFits:(CGSize)size
.
CGRect adRect = CGRectZero;
CGRect contentFrame = self.view.bounds;
adRect.size = [banner sizeThatFits:contentFrame.size];
adRect.origin.y = CGRectGetHeight(contentFrame) - CGRectGetHeight(adRect);
banner.frame = adRect;