There are creatures in DND 5e with abilities that allow them to move into spaces as small as 1-inch without squeezing: Air-Form from Air Elementals, Water-Form from Water Elementals, Fire-Form from Fire Elementals, and Amorphous from Oozes.
Therefore can these creatures move through spaces smaller than 1-inch if they use the rules for squeezing? If so what’s the smallest sized space that they’d be able to squeeze through in your option?
For reference the rules for squeezing are:
Squeezing into a Smaller Space
A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that’s only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it’s in the smaller space.