I measured the inside first since I have a lot of ribbon spools, and the 12 by 6 inch floor fits way more than I expected. Six of them stacked on my closet shelf without wobbling, which is the part I was most worried about.
The latching lids actually snap shut, which matters when I'm hauling yarn and fabric scraps between rooms. Inner space of 12 by 6 inches fits my rotary cutter and rulers without cramming. Six of them stacked in the closet and I finally reclaimed a shelf.
I was worried the 6-pack would take up too much room, but they stack so securely that my entire shelf looks twice as organized now. The 13-quart size is perfect for my yarn stash and the clear lids make it easy to spot colors at a glance.