it's been a few months but figured I'd chime in. if i understand the question correctly i believe auto sizing is a function of the popup engine itself. for instance lytebox has an autoresize option in lytebox.js that is enabled by default:
this.autoResize = true; // controls whether or not images should be resized if larger than the browser window dimensions
highslide appears to have the option enabled as well (highslide.js):
allowSizeReduction: true, // allow the image to reduce to fit client size. If false, this overrides minWidth and minHeight
fancybox seems to have this option enabled as well. It does appear that slimbox and slimbox2 don't have this feature though (
source
). I didn't look at all the scripts but it seems that the one's with this feature have it enabled by default. So try switching the popup engine.
They seem to work in firefox, but I can't attest to their functionality in a mobile browser.
FYI: There are many other options available in most of the engine js files that can be adjusted such as borders, transition duration and type, default box sizes, slide show options, navigation buttons etc.
