A Medium fits a mounted 70-200 L vertically. The width is the same,
A Small fits a Pro body/ 70-200L and 16-35L (or similar) broken down, it is 20cm vertical same width, there is some left over room for smaller items... cables etc. Ideally the top row has the tele and the wide and body on the lower. All arranged by dividers... so you can change that according to your needs.
They sound pretty much the same size, but its hard to visualize. I can't wait for some photos to see.
Also for the ICU, the straps on the side, they are supposed to go through the rings on the inside of the pack along the frame? Does that help the pack in anyway, or is it just to hold it in place better?
Also neat spot for the rain cover