Learn Portuguese (Portugal): Eu acho que o salão tem um janelão. - I think that the large hall has a large window.

When you add the augmentative suffix -ão to a feminine noun to mean something is unusually large, the new word almost always becomes masculine! So, a sala becomes o salão and a janela becomes o janelão.

Does salão always mean "large room", or is it a specific vocabulary word?

It functions as both! While it literally means "large room" (from sala + -ão), salão has become the standard dictionary word for a "hall", "salon", or "ballroom" (like a hair salon: salão de cabeleireiro). Janelão, on the other hand, is just a descriptive augmentative for a very large window.

I've been learning the subjunctive mood. Why is it tem (indicative) and not tenha (subjunctive) after acho que?

In Portuguese, expressing a positive belief with verbs like achar que (to think that) takes the normal indicative mood, because you are stating what you consider to be a reality. You only trigger the subjunctive if you make it negative to express doubt: Não acho que o salão tenha um janelão (I don't think the hall has a large window).

What happens if I want to make these augmentative words plural?