No, we only know magical items can be resized and ordinary items can't be resized. The rest was a 'that would be useful' theory on my part. I can't see Translyvito giving out magical coffins for comfort only, I'm thinking they could be some sort of armor. Like the white grease parody jacket Vinny wore before being sent to the gap-fight.
It sounds like you're assuming that there is some substantial cost to make things magical, and then an additional cost for each function, such that it would be a waste to spend the money to make a magical item only to have the item resize.
I think it's more likely that there's just a cost per function, and the magical / non-magical attribute is just applied to the item based on its functions. In other words, the focus is on the fact that the Transylvito coffins are magical because they resize, not that they resize because they're magical.
For example, it might cost 5 shmuckers to make make a coffin, 8 shmuckers to make a magical coffin that resizes, but 400 shmuckers to make a magical coffin that can detect magic. You could get 50 resizing coffins for the price of one resizing detecting coffin.