Sizes

Broad is another word for wide. The man in the picture has a broad chest.

A dwarf is a small person, born with a condition that makes them unusually small. Most dwarves have short arms and legs and facial features that are distinctive.
A dwarf of any kind of plant or animal is a smaller variety of that plant or animal. Dwarf forms of animals are often kept as pets.

Gigantic is another word for huge.

Large is another word for big. The cat in the picture is large.

Something is long if it goes for a considerable way in one direction. Something goes for a long time if it happens for an extended period.
The lizard in the picture has a very long tail.
The opposite of long is short.

Medium is a way to describe something that is a size in between small and large.

A gap between two things is narrow if the two things are quite close together and there is not a lot of room between them. The street in the picture is very narrow. The opposite of narrow is wide.

Something is small if other things are much bigger than it. In the picture, the dog on the bench is much smaller than the dog on the ground.

Something is thick if it is a larger than normal distance from one side to another. Things like slices of bread are often described as thick. The picture is of some toast with a thick layer of chocolate flavored spread on it.
The opposite of thick is thin or sparse.

Something is tiny if it is much, much smaller than something of its kind should normally be. The picture is of a tiny frog.