Commentary for Berakhot 45:20

SMALL SEALED VESSELS PROTECT [EVEN INSIDE] A TENT OVER A CORPSE. Rav Ashee is proving that a vessel that has less than a tefach of space is also considered a vessel and t’filin can be placed in such a vessel. They are then thought of as being in a separate area and then can be placed on the floor. He proves his point from the fact that an earthenware vessel, even less than a tefach, that is enclosed with a lid protects food that is in it from becoming tomay. This proves that theses small vessels are legally capable of separating that which is in them from the surrounding area. There is another way of viewing this phenomenon. One can argue that the reason the food inside does not become tomay is because it is as if it is totally surrounded by material that cannot be penetrated by tumoh. An earthenware vessel does not become tomay from the outside. If so, it may be that a vessel that is smaller than a tefach is not considered a vessel at all, but as far as tuimoh is concerned the earthenware material that is wrapped around it prevents tumoh from penetrating.
However, this reasoning would not allow us to put t’filin on the floor. Wrapping the t’filin in a wrapper does not separate the t’filin from the surrounding area; it is only being in a vessel that can accomplish such a separation. Tosfos needs to show that even in as far as tumoh is concerned, it is the fact that the food is in a vessel that saves it from becoming tomay, not the fact that it is wrapped in material that resists the penetration of tumoh. And we are compelled to say that the reason a vessel protects food from becoming tomay is because of the airspace¹Tosfos uses the word אויר which literally means air as if it meant the space of the air in the vessel. As we explain in the Tosfos it is not merely the earthen material of the vessel that protects food from tumoh, but the fact that it is a vessel that consists of material and airspace. of the vessel. For if there was no airspace in the vessel they would not protect from tumoh at all. For if there was no airspace in the vessel and the material of the vessels was wrapped around the food like a wrapper that would be the equivalent of food that was kneaded with mire that do become tomay when in the tent of a corpse.
It is clear that it is not the earthen material that protects the food from becoming tomay, because we see that when we have the material wrapped around the food in a non-vessel form, the food does become tomay. It is the element of being in a vessel that protects the food from becoming tomay. We can now understand Rav Ashee’s proof from the food that is protected from tumoh by being in a vessel. Since it is because it is in a vessel that it is protected from tumoh, so too, the t’filin when placed in a vessel are separate from the surrounding area and they may be placed on the floor.

Even less than a handbreadth in size.