It is not an Sigil issue. The best way to use the maximum screen space is to use a wrapper. The code for that is also used for covers in Sigil. That is the same answer you got in the ePUB forum.
Also, you have a margin specified for the div containing the image, so the complete screen will never be used. That is even besides the potential margin for the body and html tags.
Another point, the auto value does not really work for a lot of readers It will be interpreted as being 0 (this in accordance to the ePUB specifications).
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