Learn Portuguese (Portugal): O restaurante é muito grande.

É comes from the verb ser, which we use for permanent or defining characteristics. Since the physical size of a building doesn't change from day to day, being big is a permanent trait of the restaurant. We would use estar (like está) for temporary states, like if the restaurant was temporarily closed or crowded.

How do I know if restaurante is masculine or feminine since it ends in an '-e'?

The word restaurante is masculine, which we can easily tell from the masculine word for 'the' (O) at the beginning of the sentence. Nouns ending in '-e' in Portuguese can be either masculine or feminine, so it is always best to memorize them alongside their article.

Since restaurante is masculine, shouldn't the adjective for 'big' end in an '-o'?

No, grande is a uniform adjective, meaning it doesn't change for gender. It stays grande whether it is describing a masculine noun (like o restaurante) or a feminine noun (like a casa). It only changes if the noun is plural (becoming grandes).

Would the word muito ever change endings to match the noun?