Here is a screen grab of a project of mine with AdMob ad's, running on my Galaxy Tab 2 7.0. I measured the advert in paint, an it's definitely 320 x 50.
I also have a Galaxy Y (The lowest of the low spec Androids) with a 480 x 320 display; when running the same the advert fills most of the top of the screen, allowing just enough space for the close button. (Would add screen grab of that but my battery is flat, *sigh*)
When I run a project in portrait mode on the Y, the advert fills the whole screen width, again with a height of 50px and I allow for this when designing the UI.
I also at times hide the advert if there is no way of arranging the UI around it (Notably when the On screen keyboard is visible)