So, there is a shop that sells canvasses somewhere in the game. However, the details about how big each of these are are only available if you have the object; the details aren't in the item description.
An answer was given to me that small is about half a meter long, medium a meter, and large about 1.33 meters. It'd be really helpful if these bits of size info were added to their descriptions, this way we'd know :)
