Learn Portuguese (Portugal): Aqueles hotéis são grandes?

For words ending in -l, you form the plural by dropping the -l and adding -is. The accent mark on the é is added to keep the stress on the final syllable. Without it, the pronunciation rules would naturally shift the stress to the wrong part of the word.

Is grandes a masculine or feminine word?

It acts as both! The singular adjective grande ends in -e, meaning it is gender-neutral and doesn't change based on whether the noun is masculine or feminine. To make it agree with the plural noun hotéis, you simply add an -s.

Could I say Esses hotéis instead of Aqueles hotéis?

You could, but it changes the meaning slightly based on distance. You use esses when the things you are talking about are close to the listener. You use aqueles when the things are far away from both you and the listener, like saying "those hotels over there".

Why do we use são instead of estão here?