Learn Portuguese (Portugal): O meu avô dorme naquele cadeirão no salão. - My grandfather sleeps in that armchair in the large hall.

Yes! The word cadeirão comes from cadeira (chair), so it literally means a big chair, which we call an armchair. Similarly, salão comes from sala (room), meaning a very large room, lounge, or hall.

Since cadeira and sala are feminine words, why does the sentence use the masculine words naquele and no before them?

When you add the augmentative suffix -ão to a feminine noun, the new word almost always becomes masculine. Because of this gender swap, a cadeira becomes o cadeirão, and a sala becomes o salão. That is why we use the masculine contractions naquele (em + aquele) and no (em + o).

How would I make cadeirão and salão plural if there were multiple armchairs or halls?

Most augmentative words ending in -ão form their plural by changing to -ões. To talk about multiple armchairs, you would say cadeirões, and for multiple halls, you would say salões.

Why does the sentence start with O meu avô instead of just Meu avô?