Cái áo sai kích cỡ.

Cái is a classifier. In Vietnamese, many countable nouns are commonly used with a classifier when you are talking about a specific item.

So:

• áo = shirt / top / garment
• cái áo = a/the shirt, with a natural noun phrase shape

In this sentence, cái áo sounds like you are referring to a particular shirt. English uses articles like a or the, but Vietnamese uses classifiers instead of articles.

A more literary or slightly more elegant classifier here can be chiếc:

• chiếc áo

Both are possible, though cái áo is very common in everyday speech.

Because Vietnamese often leaves out the copula to be when the predicate is an adjective or a descriptive phrase.

So:

• Cái áo sai kích cỡ.
  = The shirt is the wrong size.

This is normal Vietnamese structure.

You usually do not need là here.
Là is more often used when linking one noun to another, as in:

• Anh ấy là bác sĩ. = He is a doctor.

But with descriptions like wrong, big, small, beautiful, expensive, etc., Vietnamese often has no equivalent of is:

• Áo đẹp. = The shirt is beautiful.
• Áo đắt. = The shirt is expensive.
• Áo sai kích cỡ. = The shirt is the wrong size.

Áo is a general word for an upper-body garment. Depending on context, it can be translated as:

• shirt
• top
• jacket
• sometimes more generally garment

If you want to be more specific, Vietnamese often adds another word:

• áo sơ mi = dress shirt / button-up shirt
• áo thun = T-shirt
• áo khoác = jacket
• áo len = sweater

So in Cái áo sai kích cỡ, the exact English translation depends on context. If the meaning shown to the learner is The shirt is the wrong size, that is a perfectly reasonable translation.

Literally:

• sai = wrong / incorrect
• kích cỡ = size

So sai kích cỡ means wrong size or incorrect size.

In natural English, you would usually say:

• The shirt is the wrong size.
• The shirt has the wrong size. is less natural in English.

In Vietnamese, sai is doing the job of saying something is incorrect or not matching what it should be.

In this sentence, it works like a predicate adjective or stative verb: it describes the state of the shirt.

Vietnamese does not always separate adjectives and verbs the same way English does. A word like sai can function very naturally as the main predicate without any extra word:

• Câu trả lời sai. = The answer is wrong.
• Địa chỉ sai. = The address is wrong.
• Cái áo sai kích cỡ. = The shirt is the wrong size.

So the easiest way to think about it is: sai is the descriptive core of the predicate.

All three can refer to size, but they are a little different in tone and usage.

• kích cỡ = size, dimensions; fairly standard and a bit formal
• cỡ = size; shorter and very common
• size = English borrowing; extremely common in shops and everyday speech

So these are all possible:

• Cái áo sai kích cỡ.
• Cái áo sai cỡ.
• Cái áo không đúng size.

In conversation, many speakers would actually say size. In more careful or formal Vietnamese, kích cỡ is a good choice.

Vietnamese usually puts the subject first and the description/predicate after it.

So the structure is:

• Cái áo = the shirt
• sai kích cỡ = is the wrong size

That gives:

• Cái áo sai kích cỡ.

This is similar to many simple Vietnamese sentences:

• Cái áo đẹp. = The shirt is beautiful.
• Cái áo rộng. = The shirt is wide/loose.
• Cái áo đắt. = The shirt is expensive.

So the order is very natural: noun first, description after.

By itself, cái áo does not map exactly to just one English article.

Depending on context, it can mean:

• the shirt
• a shirt
• sometimes even that shirt or this shirt, if the situation already makes it clear

Vietnamese does not have articles like a/an/the. Context does that work.

If you want to be explicit, you can add a demonstrative:

• cái áo này = this shirt
• cái áo đó/kia = that shirt

So:

• Cái áo này sai kích cỡ. = This shirt is the wrong size.

Literally, it means the shirt is the wrong size. That can imply a few related situations:

• the wrong size was chosen
• the wrong size was sent
• the size label is not the one needed
• the shirt is not the correct size for someone

If you specifically want to say it does not fit, Vietnamese often uses more direct phrases:

• Cái áo này không vừa. = This shirt doesn’t fit.
• Cái áo này quá chật. = This shirt is too tight.
• Cái áo này quá rộng. = This shirt is too loose / too big.

So sai kích cỡ is a bit more about incorrect size, while không vừa is more directly about fit.

It is understandable and acceptable, especially in a shopping, returns, or product context.

But in everyday speech, a native speaker might more often say:

• Cái áo này sai kích cỡ.
• Cái áo này không đúng kích cỡ.
• Cái áo này không đúng size.
• Cái áo này không vừa.

The version with này often sounds more natural in conversation because speakers are usually pointing to or talking about a particular shirt:

• Cái áo này sai kích cỡ. = This shirt is the wrong size.

So the original sentence is fine, but in real-life speech it often appears with a word like này for clearer reference.