Fixed width lightboxes?

You will have to change the plugin code in photonic/include/scripts/photonic.js:

Look for this code:

$j('a.launch-gallery-colorbox').each(function() {
		$j(this).colorbox({
			opacity: 0.8,
			maxWidth: '95%',
			maxHeight: '95%',
			title: photonicLightBoxTitle(this),
			slideshow: Photonic_JS.slideshow_mode,
			slideshowSpeed: Photonic_JS.slideshow_interval
		});
	});

Replace maxWidth and maxHeight with width and height respectively.

You have to make a similar change in photonic/extensions/Photonic_Processor.php, where you will see this code:

else if ($this->library == 'colorbox') {
			$ret .= "<script type='text/javascript'>\$j('a.launch-gallery-colorbox').each(function() { \$j(this).colorbox({ opacity: 0.8, maxWidth: '95%', maxHeight: '95%', title: photonicLightBoxTitle(this), slideshow: Photonic_JS.slideshow_mode, slideshowSpeed: Photonic_JS.slideshow_interval });});</script>";
		}