Learn Portuguese (Portugal): Eles pintaram um muro enorme ontem.

Yes, absolutely! Saying um murão is a very natural, everyday way to say a huge wall. Using an adjective like enorme or grande is just another way to express the exact same idea. Portuguese speakers frequently switch between using augmentative suffixes and separate adjectives to describe large things.

What is the difference between muro and parede, which both mean wall?

A muro is an outdoor wall, like a brick wall around a property, a garden wall, or a standalone barrier. A parede is an interior wall or a structural wall of a building. Since the sentence uses muro, we know they were painting outside!

Why doesn't the adjective enorme end in -o to match the masculine noun muro?

Enorme is an invariable adjective, meaning its spelling doesn't change based on gender. It stays exactly the same whether it describes a masculine noun like um muro enorme or a feminine noun like uma parede enorme. It only changes to match plural nouns, where it simply adds an s to become enormes.

I sometimes mix up the past and present forms for they. How do we know pintaram is the past tense?