Το σακάκι αυτό είναι ωραίο, αλλά δεν ξέρω αν το νούμερο είναι σωστό για εμένα.

Both αυτό το σακάκι and το σακάκι αυτό are correct.

In this sentence, το σακάκι αυτό literally looks like the jacket this, but in natural English it still means this jacket.

A useful rule:

• αυτό το σακάκι = the most basic, common way to say this jacket
• το σακάκι αυτό = also correct, often a bit more pointed or specific, like this particular jacket

So the sentence could also be:

• Αυτό το σακάκι είναι ωραίο...

Both are natural Greek.

Greek uses the definite article very often, much more often than English.

So:

• το σακάκι = the jacket
• το νούμερο = the size / the number

Even when English might say just this jacket or if the size is right, Greek normally keeps the article:

• Το σακάκι αυτό
• το νούμερο

This is completely normal and not especially emphatic.

Σακάκι is a neuter noun.

That affects several words around it:

• το σακάκι → neuter article το
• αυτό → neuter form of this
• ωραίο → neuter form of nice / beautiful

So all of these match σακάκι in gender, number, and case.

Because ωραίο must agree with το σακάκι, which is neuter singular.

The adjective ωραίος changes form depending on the noun:

• ωραίος = masculine
• ωραία = feminine
• ωραίο = neuter

So:

• ο άντρας είναι ωραίος
• η γυναίκα είναι ωραία
• το σακάκι είναι ωραίο

Usually σακάκι means jacket, especially a smart jacket, blazer, or suit jacket.

Depending on context, it may not mean every kind of outerwear. For example:

• a blazer or suit jacket: very natural for σακάκι
• a winter coat: usually a different word, like παλτό
• a casual zip-up jacket: sometimes another word may be more natural depending on the type

In this sentence, since the speaker is talking about the size, σακάκι is best understood as jacket or blazer.

Yes, νούμερο literally means number, but in clothing contexts it often means size.

So:

• το νούμερο του παπουτσιού = the shoe size
• το νούμερο in a shop can mean the size

In this sentence:

• δεν ξέρω αν το νούμερο είναι σωστό = I don’t know if the size is right

That is very natural Greek.

Because σωστό agrees with το νούμερο, which is also neuter singular.

So:

• το νούμερο είναι σωστό means
• the size is correct/right

Again, adjective agreement matters:

• σωστός = masculine
• σωστή = feminine
• σωστό = neuter

Here αν means if / whether in an indirect question.

So:

• δεν ξέρω αν... = I don’t know if / whether...

In English, whether is often slightly more formal, but Greek αν works very naturally here.

Example:

• Δεν ξέρω αν έρχεται. = I don’t know if/whether he is coming.

So in your sentence:

• δεν ξέρω αν το νούμερο είναι σωστό = I don’t know if the size is right

Yes. ξέρω means I know, and δεν makes it negative.

So:

• ξέρω = I know
• δεν ξέρω = I don’t know

This is the standard everyday way to say it.

The full clause:

• δεν ξέρω αν το νούμερο είναι σωστό means
• I don’t know if the size is right

Because Greek uses για here to express the idea for me or for my body / my fit.

So:

• σωστό για εμένα = right for me
• more naturally in English: the right size for me

If you only said εμένα here, it would not sound right in standard Greek. The preposition για is needed.

They mean the same thing: for me.

• για μένα = more common in everyday speech
• για εμένα = fuller form, sometimes slightly more careful or emphatic

So the sentence could also be:

• ...αλλά δεν ξέρω αν το νούμερο είναι σωστό για μένα.

That would sound very natural too.

Yes, literally it means right/correct for me, so it can refer to suitability in a broader sense. But because the sentence specifically mentions το νούμερο, the meaning is clearly about size/fit.

So here:

• το νούμερο είναι σωστό για εμένα really means
• the size is right for me
• it’s the right size for me

Greek word order is fairly flexible, although some versions sound more natural than others.

The original:

• Το σακάκι αυτό είναι ωραίο, αλλά δεν ξέρω αν το νούμερο είναι σωστό για εμένα.

A very natural alternative:

• Αυτό το σακάκι είναι ωραίο, αλλά δεν ξέρω αν το νούμερο είναι σωστό για μένα.

You could also hear:

• ...αλλά δεν ξέρω αν είναι σωστό το νούμερο για μένα.

That said, the original sentence is perfectly normal.

A very natural spoken version would be:

• Αυτό το σακάκι είναι ωραίο, αλλά δεν ξέρω αν το νούμερο είναι σωστό για μένα.

That version sounds a little more conversational because:

• αυτό το σακάκι is slightly more common in speech
• για μένα is more everyday than για εμένα

But the original sentence is absolutely correct and natural too.