The way I normally format full page ebook images in an epub is by using inline styling that uses percentage values within the <img> tag in the html:
Example:
<p class="center"><img alt="image001" src="../Images/YOUR_IMAGE.jpg" style="width: 100%; height: auto;"/></p>
The above code will ensure that the displayed image is at maximum page width and height in Sigil.
If you have alot of ebook images to convert and you want the above done automatically then you could just run Sigil's
CustomCleanerPlugin plugin and be sure to select
"Reformat smaller ebook images to preserve image size" in the options dialog. This choice will convert and resize all ebook images to a percentage of current device screen size. This also means that any ebook images that are greater than page size will automatically be resized to 100% page width/height.
And if you are formatting your epub for Kindle upload only, then I would advise you to just run the
AddKindleMediaQueries Sigil plugin. This plugin will dual format all ebook images using media queries to ensure that all images are dispalyed properly on KF8 devices(with percentage values) and on older KF7 devices(with pixel values).