How to tell size category of an object?

Question

Artificers "Magical tinkering" ability says

You’ve learned how to invest a spark of magic into mundane objects. To use this ability, you must have thieves’ tools or artisan’s tools in hand. You then touch a Tiny nonmagical object as an action and give it one of the following magical properties of your choice:

Use of capital letter Tiny suggests that this is a mechanical term, not just a common language usage of the word, but as size categories are usually used only in reference to creatures, I would like to know what objects can be impacted with this ability. Can I make a cool symbol on a shield? Make a sword emit a sound?

This question suggests that size category for objects can't be determined, but as it was focused on a specific magical weapon I hope to get better answers here

What size is a Solar's Sword?

I'm aware that I can just craft some rule of thumb with my GM and probably will end up doing this, but first I want to check if there is an official answer.

Answer 1

A Tiny object is comparable in size to a normal bottle or lock

There is no hard definition for the weight or dimensions of a Tiny object, but based on examples cited in the rules it should likely not be larger than maybe 2 feet in any direction, and weigh not more than 2 pounds.

A sword specifically would already be a Small object.

Tiny Objects

The DMG lists example objects along with the size category on p. 247. It has

Tiny (bottle, lock)

The bottle weighs 2 pounds, the lock one pound.

Quaal's Feather Token is a tiny object "that looks like a feather", so a typical feather is another example.

A Tiny object must be smaller than a Small object, so let us look at a few small objects as an upper bound.

Small Objects

Small (chest, lute) (DMG p. 247)

As a Small object, the typical chest is described as containing 12 cubic feet (p. 153 PHB). The chest from Leomund's Chest is likewise able to contain 12 cubic feet, and has dimensions of 2 x 2 x 3 feet.

A lute is about 3 feet long, 1 foot wide. It weighs 2 pounds in the PHB.

In the Monster Manual, the Flying Sword is a Small Construct, of the same size as a normal sword, because it has False Appearance:

While the sword remains motionless and isn't flying, it is indistinguishable from a normal sword.

A normal longsword can be a as short as about 3 feet, with width and depth just a few inches. It weighs only about 2-3 pounds (3 in the PHB).

Answer 2

A Tiny object is approximately the same size as a Tiny creature.

The usual size categories for creatures apply to objects as well. The Dungeon Master’s Guide has rules for object statistics that rely on the usual size categories for creatures:

The Object Hit Points table provides suggested hit points for fragile and resilient objects that are Large or smaller.

The table then gives hit point values for objects sized Tiny up to Large. Now, I have been a bit ambiguous about the exact size of Tiny, because it is a range - the creature size categories suggest anything less than 2.5 x 2.5 feet, but the table seems to treat objects similarly to creatures. The trouble here is that with creatures, their size is about the space they control, not just their physical dimensions. Since objects don’t move, your mileage may vary when actually comparing an object’s size to a similarly sized creature. Finally, since medium and small are both 5x5 feet, the distinction between them may be a bit blurry.