Learn French: Il y a un chaos énorme dans le magasin. - There is enormous chaos in the store.

You would replace un chaos énorme with tellement de chaos or tant de chaos. Then you add que (that) followed by a consequence. For example: Il y a tellement de chaos dans le magasin que je pars (There is so much chaos in the store that I am leaving).

How is chaos pronounced? Does it start with a "sh" sound like most French words with "ch"?

No, chaos is an exception in French. The ch makes a hard "k" sound, and the final s is silent. It is pronounced roughly like "kah-oh".

Why does énorme come after chaos instead of before it?

Most descriptive adjectives in French go after the noun. While some common adjectives for size go before the noun (like petit or grand), énorme is an exception to that pattern and usually follows the noun it describes.

Why use dans le magasin instead of au magasin?

Dans emphasizes being physically inside the enclosed space of the building. If you were just talking about the store as a destination or general location, you would use au (for example, Je vais au magasin meaning "I am going to the store").