Did I miss something here? Isn't that what you want?
Define if you want the map to automatically adapt to your screen size. Choose between:
Scale map view to fit browser size: The size of the map automatically adapts to the
size of your screen. Choose this method if the map is shown on various screens with
different resolutions so that it uses the available space in the browser window in the
best possible way.
Do not auto-scale map view: The map uses always the size that you define above.
It won't always do the 100% width, but it does the height. Having said this - it would be nice if you could chose to use either the height or the width - but that's a different story. It still scales for sure. Using that all the time myself.