Какой размер этой рубашки?

Because какой means which / what kind of, and Russian uses it when asking about a category or type.

Here, размер is not just any random thing; it is a category with possible values like small, medium, 42, 46, and so on. So Russian asks, literally, Which size is this shirt?

Using что would sound wrong here, because что asks what? in a more general sense.

• Какой = what kind of / which
• размер = size
• этой = of this / this in a form that matches the noun it describes
• рубашки = shirt in the genitive singular form

Word-for-word, the structure is close to:

What size of this shirt?

Natural English does not say it that way, but that is how the Russian grammar is built.

Because after размер, Russian often uses the thing being measured in the genitive case.

So:

• рубашка = shirt
• размер рубашки = the size of the shirt

That is why the sentence uses рубашки: it means of the shirt.

Because этой has to agree with рубашки, and рубашки is in the genitive singular.

The noun рубашка is:

• feminine
• singular
• here, in the genitive case

So the demonstrative this must also be feminine singular genitive:

• nominative: эта рубашка = this shirt
• genitive: этой рубашки = of this shirt

In Russian, the verb to be is usually omitted in the present tense.

So where English says:

What size is this shirt?

Russian simply says:

Какой размер этой рубашки?

This is completely normal. Russian speakers do not need a present-tense is here.

It is in the nominative masculine singular form.

That is because it agrees with размер, which is:

• masculine
• singular
• nominative

So:

• какой размер = what size

If the noun were different, какой would change too. For example:

• какая рубашка = what kind of shirt
• какое пальто = what kind of coat
• какие размеры = what sizes

Yes. Russian word order is more flexible than English word order.

The neutral version is:

Какой размер этой рубашки?

But you may also hear:

• Этой рубашки какой размер?
• Какой у этой рубашки размер?

These can sound slightly different in emphasis, but they are all understandable. The version with у is especially common in everyday speech.

Yes. A very common alternative is:

Какой у этой рубашки размер?

Literally, this is something like:

What size does this shirt have?

Another common pattern is:

Эта рубашка какого размера?

Literally:

This shirt is of what size?

All of these are natural. The original sentence is correct and clear, but in a shop you may often hear one of these alternatives.

The stress is:

Како́й разме́р э́той руба́шки?

Approximate pronunciation:

• Какой = ka-KOY
• размер = raz-MYER
• этой = EH-tay
• рубашки = roo-BASH-kee

The stressed syllables are important in Russian, so it helps to learn them early.

No, in normal usage it simply asks for the shirt’s size.

Even though какой can sometimes mean what kind of, with размер it naturally means which size / what size.

So a Russian speaker understands this as a question about size information, not as a strange question about the type of size.

Because you are asking for one size value for one shirt.

So Russian uses the singular:

размер = size

If you were asking about several possible sizes, then plural might appear in a different sentence, for example:

Какие размеры есть у этой рубашки?
= What sizes are available for this shirt?

But in the given sentence, singular is exactly right.