Unfolding Animate Objects

Question

I'm playing a bard in a campaign with my friends in D&D 5e. One of the biggest goals of my character is to fly. Now my character is level 9 and has finally reached the animate objects spell. I know that if we place our character on a Large object in Animate objects, our character can mount and even fly, but an interesting question came to my mind. Imagine a metal cube of small size. Let's say this small cube can be unfolded. As the cube is opened, its size increases and it is considered Large. Accordingly, should we use the small object stat block or the large object stat block in animate objects for this object?

Notes:

1. I don't want to use a rug with my animate objects
2. I don't want to waste my bards magical secrets with fly or levitate

Answer 1

This has several issues

Animate Objects says:

Choose up to ten nonmagical objects within range that are not being worn or carried. Medium targets count as two objects, Large targets count as four objects, Huge targets count as eight objects. You can't animate any object larger than Huge.

You are assuming that if you had some kind of fold-out cube, that the cube would change its size from Small to Large, if you change its form. That is not the case: it may have a different geometric form then, but it still is a Small object, as size is an inherent property of the object. A rope as an object does not change its size, just because you unroll it. And an once the object is animated, it is a creature, and nothing in the spell says the creatures can change their size.

Thus they cannot be used as a mount for a Small or Medium creature, as the Mounted Combat rules (p. 198, PH) say:

A willing creature that is at least one size larger than you and that has an appropriate anatomy can serve as a mount

If you could change the size of the objects by changing their form after animating, this would give you a way to cheat on the amount of Medium or Large objects that a single casting could animate, so that will not work.

Even if your DM agreed to changing their size, the objects likely would stop being animated when they change the size, as they are now not of the size you could animate in that number any more (the rules on how spells react if conditions required for them to change are not very clear, it is commonly assumed they are supressed if the target becomes illegal, but in the end it is a DM call).

Answer 2

A creature can't just change its size category

The animated objects are considered creatures, we learn that because the spell refers to them as constructs, which is a creature type and also because they get values for the 6 ability scores assigned to them. Each creature has a size it is categorized in. In the case of this spell, those sizes are assigned when the spell is cast. A size category is generally used to determine a creature's hit dice (unless it's a player character) as well as how much space it typically takes up on a battlefield.

Now your question is essentially: Does a creature's size category change when it expands itself? Imagine the following scenario: You enter a shop, but the human shopkeep (size medium) is ducking down behind the counter trying to hide from you. Does this make him a a small creature, because his body is half as tall while ducking and then make him revert to being medium again when he decides to stand up again? No, the shop keep is always considered a medium creature, it doesn't matter if he stands, sits, lies on the ground or holds his hands above his head.

The same is true for your unfolding animated object. If it can expand to become of size large, that can only be because it was considered a large creature in the first place, i.e. when the spell was cast and this tells us it is just right now in a position that makes it take up less space then it would in its unfolded form, just like the ducking shop keep. Ask yourself: What is this object's "normal" form? If it is being your large flying mount, then it is a large creature, that you can just also fold together to put it in your pocket. Even if it was in that folded form when the spell was cast on it.

The folded object is squeezing

There is a rule for creatures that don't become a smaller size, but take up less space, it's called squeezing. Any creature can move through a space that is one size smaller than it. This decreases the amount of space the creature takes up on a battle field (except for medium creatures) and makes it suffer some negative effects such as each step costing it more of its movement.

As stated a creature can typically only squeeze into a size that is 1 size smaller. Your DM might however rule that your animated object is so flexible it can squeeze from Large all the way down to Small.