Learn Portuguese (Portugal): Aquela nuvem é muito grande.

In Portuguese, there are two words for "that". We use essa for something close to the person you are talking to, and aquela for something far away from both of you. Since a cloud is high up in the sky and physically distant from everyone, aquela is the correct choice.

Nuvem doesn't end in "-a". How can we tell it is a feminine word?

Words ending in "-m" can be tricky. While some are masculine (like homem, meaning man), many common words ending in "-em" are feminine, such as a viagem (the trip) and a nuvem (the cloud). You can tell it is feminine in this sentence because it pairs with the feminine demonstrative aquela instead of the masculine aquele.

Clouds constantly change shape. Why do we use é (from ser) instead of está (from estar)?

This is a classic tricky spot! At earlier levels, you often learn that ser is for permanent things and estar is for temporary things. However, a better rule for physical traits like size, color, or shape is that they define what the object fundamentally is. Even though the cloud itself might disappear soon, its physical dimension as an object requires ser.

Since nuvem is feminine, why doesn't the adjective grande change to match it?