Esta área é grande.

Because área is a feminine singular noun, and the demonstrative has to agree with it.

• este = masculine singular
• esta = feminine singular

So:

• este livro = this book
• esta área = this area

In Portuguese, words that describe or point to a noun often have to match its gender and number.

Yes. Área is a feminine noun: a área.

That is why the sentence uses:

• esta área
• not este área

A learner may expect a pattern like Spanish, where some feminine nouns that begin with a stressed a use el in the singular, but Portuguese does not do that here. In Portuguese, it is simply a área.

The accent in área shows the stressed syllable: Á-re-a.

It is a proparoxytone word, meaning the stress falls on the third-to-last syllable. In Portuguese, all proparoxytone words take a written accent.

So the accent is not optional; it is part of the correct spelling.

É is the verb ser in the 3rd person singular present: is.

The accent is important because it distinguishes é from e:

• é = is
• e = and

So:

• Esta área é grande. = This area is big.
• área e grande would be wrong here, because e is not a verb.

Portuguese usually uses ser for something seen as an inherent or defining characteristic, and estar for a temporary state or condition.

Here, grande describes the size of the area as a characteristic, so é is the normal choice:

• Esta área é grande. = This area is big.

If you said Esta área está grande, it could sound more contextual, like the area has become large or seems large in a particular situation. That is much less neutral.

Because grande is one of many Portuguese adjectives that has the same form for masculine and feminine in the singular.

Compare:

• o prédio é grande
• a área é grande

The adjective changes only for number here:

• grande = singular
• grandes = plural

So:

• Esta área é grande.
• Estas áreas são grandes.

In Portuguese, adjectives often come after the noun, especially in basic, neutral descriptions.

So área grande is the normal structure for big area.

In a full sentence:

• Esta área é grande.

You can sometimes place adjectives before the noun, but that can sound more literary, emphatic, or can slightly change the nuance. For a straightforward sentence like this, the usual order is exactly what you see.

No. Esta already identifies the noun, so no article is needed.

Portuguese normally says:

• esta área
• not a esta área in this kind of subject phrase

The form a esta área can exist, but then a is usually a preposition meaning to, not a definite article:

• Refiro-me a esta área. = I am referring to this area.

So in Esta área é grande, nothing is missing.

Yes, depending on the context.

Traditional grammar makes a distinction like this:

• esta = this, near the speaker
• essa = that, near the listener or already mentioned
• aquela = that over there, farther away

However, in everyday Brazilian Portuguese, the difference between esta and essa is often weaker, and essa is very common in speech. So:

• Esta área é grande. sounds correct and can feel a bit more careful or written.
• Essa área é grande. is also very natural in many spoken situations.

A rough English-friendly pronunciation is:

EH-sta AH-re-a eh GRAHN-djee

A few notes:

• esta = EH-sta
• área has stress on the first syllable: AH-re-a
• é sounds like eh
• grande in many Brazilian accents sounds close to GRAHN-djee

This is only an approximation, but it helps you hear where the stress goes.