Copy /kleo/page-parts/portfolio-masonry.php to your child theme.  Search for aq_reize.  There should be two of them; one for sliders and one for single images (default).  Change those lines, replacing $img_width and $img_height to whatever you want.  This obviously wont effect other post types like video.
COPY CODE
$image = aq_resize( $slide, $img_width, $img_height, true, true, true );
$image = aq_resize( $slide, 300, 150, true, true, true );
 
If you want to change the way it crops or anything else, read about it on the aq_resize wiki: https://github.com/syamilmj/Aqua-Resizer/wiki
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