Learn Hardcore Spanish (Latin America): El libro es grande.

In Spanish, nouns have genders. Libro is a masculine noun, so it takes the masculine article el. If it were a feminine noun like casa, you would say la casa.

Why do we use es instead of está?

Es is the verb ser, which is used for permanent or inherent characteristics, like size or color. Está (from estar) is generally used for temporary states or locations. Since being large is considered a defining quality of the book, es is the correct choice.

Why does grande come after the noun libro?

In Spanish, adjectives typically follow the noun they describe. You will see exceptions or variations, but the default pattern is noun + adjective, as in El libro es grande.

Can I say El gran libro instead of El libro grande?

Yes, you can. Putting gran before the noun changes the nuance. El gran libro can imply importance or greatness. El libro grande simply highlights the physical size.