Learn Portuguese (Portugal): Há um casarão enorme naquela rua.

When you add the augmentative suffix -ão to a feminine noun, the new "supersized" version often changes to masculine. So, the feminine a casa (the house) becomes the masculine o casarão (the big house / mansion). You will see this happen with other words too, such as a porta (the door) becoming o portão (the gate).

Where does the 'r' in casarão come from? Shouldn't it just be casão?

When adding -ão to words, Portuguese often inserts a consonant (like -z- or -r-) before the suffix to make it flow better when spoken. While casão technically exists, casarão is the standard, natural-sounding word used for a mansion or a very large old house.

If casarão already means a big house, why add the adjective enorme (huge)?

It might seem redundant, but doubling up on size descriptions is a very common and natural way to add emphasis in Portuguese. Using the augmentative -ão alongside an adjective like enorme or grande just stresses how incredibly massive the house really is.

Why do we use naquela (in that) to say "on that street"?