Hi Sam -There were some changes a couple releases ago that affected slider sizes and scaling. My advice is to decide first how wide and how tall you want your sliders to be. (Width setting are controlled in CBOX settings.) The theme will scale down, but not up – but I would not depend on automatic scaling, and instead use some image editing software to get your picture the exact dimensions that you want. For me, 670 x 345 seems to work pretty well (totally arbitrary though). Then, when you get your images from Flickr CC (or wherever) choose to download a version that you can scale appropriately using the resizing features of your software. (You might want to crop some superfluous areas.) When you are ready to set your slider’s featured image, make sure it is your chosen width and height and your should have good results.
Bottom line is to get all your slider featured images the same width and height.
see also – https://commonsinabox.org/documentation/cbox-slider
Hope this helps.