Hi,
MRAID defines two of the sizes you mention.
* Screen - this is the physical size of the device screen, for example 320x416 on the old iPhones in portrait mode. You would access this size by using getScreenSize.
* Container - this is the size of the container provided by MRAID. The size depends on the state, the device, and the app. For example, the container size may be 320x50 for iphones during the default state and 320x416 during the expanded state. You would access the current size by using getCurrentPosition, the default-state size with getDefaultPosition, and the max size allowed by the app with getMaxSize.
This one is not really defined because it is a synonym
* Webviewer - this is really just another name for the container since the container implements a webviewer in order to display HTML ads
And then the ad unit size is controlled by the creative designer.
* Ad creative - The HTML could say something like <table width="1000px"> which means that the DOM size of the table is 1000px. This works in MRAID just like it works on the web -- if the page/ad is too big,there will be scrolling. If the page/ad is too small it will display from the top left corner with a white background. Of course, that's true unless there is additional CSS for centering, clipping, etc.
Thanks,
-Nathan