Learn Spanish (Spain): Es una cosa muy pequeña.

In Spanish, subject words like "it", "he", or "she" are often left out. The verb es already means "it is", so you do not need an extra word to say "it".

Why do we use es instead of está here?

Both verbs mean "to be", but we use es (from the verb ser) to describe a core trait or basic characteristic of something. Because we are describing what the thing is like, we use es.

Why does muy pequeña come after cosa? In English we say "very small thing".

In Spanish, descriptive words (adjectives) usually come right after the noun they describe. So instead of putting the description first, the sentence is built in the order of "thing very small" (cosa muy pequeña).

Why does the sentence use pequeña instead of pequeño?

Adjectives must match the gender of the noun they describe. The word cosa is a feminine noun (which is why it uses the feminine word una for "a"). Because of this, the adjective changes its ending to match, becoming pequeña.