So the question is: is there an easy (i.e. via options) way to set the whole full-screen block, including image and image caption, so that it has ~90% viewport width, ~5% margin-left and right, and of course auto height to preserve image ratio?
As long as you are referring to the normal rather than fullscreen (expanded) gallery display, then you can set the gallery's width to be 90% in the embedding code and horizontally center the gallery's div in its parent container as follows:
<!--START JUICEBOX EMBED-->
<script src="jbcore/juicebox.js"></script>
<script>
new juicebox({
containerId: 'juicebox-container',
galleryWidth: '90%'
});
</script>
<div id="parent">
<div id="juicebox-container" style="margin: 0 auto;"></div>
</div>
<!--END JUICEBOX EMBED-->No matter what the height of the gallery is, Juicebox will respect the aspect ratio of the main images as long as imageScaleMode is set to SCALE_DOWN or SCALE.
I hope this helps.