have no idea if there's a problem going from medium to large, but when I put all the ingredients needed for the medium size on the crafting table, nothing shows up - I've tried re-downloading the mod as well as exiting Minecraft and rebooting, i'm not sure what else to do.

Oh, my fault. You are using 1.13.2 right?
The crafting recipes have changed and i forgot to update them.
You now need instead of the upper middle backpack a leather to upgrade from small to medium and from medium to large.

Thank you very much - tested and working