Yes, you use the same size on Retina devices.
However, you should not set a specific size at all. If you decide to convert your application to iPad, then your ad code will suddenly stop working, as it will stretch halfway across the screen.
Use the size of the smart banner, and Admob will work for you. For example, here is some code from one of my applications that puts a banner at the bottom of the screen. Pay attention, in particular, to the use of kGADAdSizeSmartBannerPortrait, this allows you to change the size of the advertising banner.
source share