Learn German: Seine Augen sind groß.

The base word for 'his' is sein. However, because Augen (eyes) is a plural noun, we have to add the plural ending -e to match it. That gives us seine.

Why doesn't the adjective groß have an ending like große since Augen is plural?

Adjectives in German only get endings when they are placed directly in front of a noun. Because groß comes after the verb sind (are), it stays in its basic dictionary form with no extra ending.

What is the singular form of Augen?

The singular is das Auge (the eye). In German, neuter nouns that end in -e very often form their plural by just adding an -n.

How would I say 'her eyes' instead of 'his eyes'?

You would use the base word for 'her', which is ihr. Since Augen is plural, you still add the -e ending to match, making it ihre Augen.