Αυτό το σακάκι είναι πολύ στενό, θέλω άλλο νούμερο.

Yes, αυτό means this, but in Greek it is normally used together with the definite article.

So:

• αυτό το σακάκι = this jacket
• literally, it is something like this the jacket

That sounds odd in English, but it is standard Greek structure. Both words have to match the noun in gender, number, and case.

Because σακάκι is neuter singular, Greek uses:

• αυτό = neuter singular form of this
• το = neuter singular definite article

Σακάκι is a neuter noun. Many Greek neuter nouns end in -ι, and σακάκι is one of them.

That matters because the words around it must agree with it:

• αυτό το σακάκι
• στενό

All of those are in the neuter singular form.

You often learn noun gender together with the article, so it helps to memorize it as:

• το σακάκι = the jacket

not just σακάκι by itself.

Yes, στενό is an adjective meaning tight or narrow depending on context. Here it means tight.

It ends in -ό because it must agree with σακάκι, which is neuter singular.

Compare:

• στενός = masculine
• στενή = feminine
• στενό = neuter

Since σακάκι is neuter, you say:

• Το σακάκι είναι στενό. = The jacket is tight.

Here πολύ means very and modifies the adjective στενό.

So:

• πολύ στενό = very tight

This is one of the most common uses of πολύ.

Be careful: πολύ can also relate to much/a lot, depending on context. But in this sentence, with an adjective, it clearly means very.

Greek often leaves out subject pronouns when they are not needed, and in this sentence the subject is already clear:

• Αυτό το σακάκι = the thing being talked about

Then Greek simply says:

• είναι πολύ στενό = is very tight

English needs it is, but Greek does not need a separate word for it here because the noun has already been stated.

Because Greek usually does not need to say the subject pronoun unless it is being emphasized.

• θέλω already means I want
• the ending -ω tells you it is first person singular

So:

• θέλω άλλο νούμερο = I want another size

If you say εγώ θέλω, that adds emphasis, something like:

• I want another size
• As for me, I want another size

In a normal shopping situation, just θέλω is the natural choice.

Άλλος / άλλη / άλλο can mean other, another, or different, depending on context.

Here, θέλω άλλο νούμερο means:

• I want another size
• or I want a different size

Since νούμερο is neuter singular, the form is άλλο.

Compare:

• άλλος = masculine
• άλλη = feminine
• άλλο = neuter

Yes. Νούμερο literally means number, but in clothing-shop context it commonly means size.

So:

• άλλο νούμερο = another size

This is very natural Greek in a shop. In English we usually say size, but Greek often uses number for clothing and shoe sizes.

Yes, absolutely.

Both are possible:

• θέλω άλλο νούμερο
• θέλω ένα άλλο νούμερο

The version without ένα is very natural and a bit more direct, especially in everyday speech.

The version with ένα may sound slightly more explicit:

• I want another size
• literally, I want one other size

In a shop, both are fine.

The sentence has two closely connected parts:

• Αυτό το σακάκι είναι πολύ στενό
• θέλω άλλο νούμερο

Greek often links short clauses with just a comma in everyday speech and writing, especially when the relationship is obvious from context.

So the meaning is something like:

• This jacket is very tight, I want another size.

You could make the connection more explicit in other ways, but this version sounds natural and conversational.

Its basic meaning is narrow or tight, and the exact translation depends on context.

For example:

• στενός δρόμος = narrow road
• στενό παντελόνι = tight trousers
• το σακάκι είναι στενό = the jacket is tight

So the word is flexible. In clothing contexts, tight is usually the best translation.

The words that agree with the noun would change.

For example, with a feminine noun like μπλούζα:

• Αυτή η μπλούζα είναι πολύ στενή.
  = This blouse is very tight.

With a masculine noun like πουκάμισο is actually neuter, so let’s use σοφέρ? Better to use a clothing noun like παλτό, but that is also neuter. A clearer masculine example is less common in clothing, so the main point is the pattern:

• masculine: αυτός ο ..., στενός
• feminine: αυτή η ..., στενή
• neuter: αυτό το ..., στενό

Your original sentence is neuter throughout because σακάκι is neuter.

A simple pronunciation guide would be:

• Αυτό το σακάκι είναι πολύ στενό, θέλω άλλο νούμερο.
• af-TO to sa-KA-ki EE-ne po-LEE ste-NO, THE-lo AL-lo NOU-me-ro

A few useful points:

• Stress matters in Greek.
• The stressed syllables here are:
  • αυτό
  • σακάκι
  • είναι
  • πολύ
  • στενό
  • θέλω
  • άλλο
  • νούμερο

Listening and repeating whole chunks like αυτό το σακάκι and άλλο νούμερο is especially helpful.