غرفة القياس في المتجر.

A careful MSA pronunciation is:

ghurfatu l-qiyāsi fī l-matjari

In normal pause, learners often hear something closer to:

ghurfat al-qiyās fī al-matjar

A few notes:

• غ is the sound usually written gh
• ق is a deep q sound, not an English k
• In القياس and المتجر, the l of الـ is pronounced, because ق and م are moon letters

Because Arabic often uses a nominal sentence in the present tense with no written verb for is/am/are.

So:

غرفة القياس في المتجر
naturally means
The fitting room is in the store

This is very common in Arabic. If you wanted past tense, then you would normally use a verb such as كان.

Because غرفة القياس is an iḍāfa construction, often called a construct phrase.

In an iḍāfa:

• the first noun usually does not take الـ
• the second noun shows whether the whole phrase is definite

So:

• غرفة قياس = a fitting room / a measuring room
• غرفة القياس = the fitting room / the measuring room

Even though غرفة has no الـ, the whole phrase is definite because القياس is definite.

Because the word ends in ة (tāʾ marbūṭa).

• When a word with ة is said by itself, it is usually pronounced -a
• When it is connected grammatically to another word, especially in iḍāfa, it is pronounced -at (or -tu / -ta / -ti with case endings)

So:

• by itself: غرفة → ghurfa
• in غرفة القياس: ghurfat al-qiyās

This is a very important pattern in Arabic.

It is an iḍāfa: noun + noun.

Here:

• غرفة = room
• القياس = measuring / fitting

Together, the phrase means something like room of measuring, which is the Arabic way of forming this kind of expression.

In an iḍāfa:

1. the first noun does not take الـ
2. the second noun is in the genitive
3. the whole phrase acts like one unit

If you fully vowel the sentence as a complete nominal sentence, it would normally be:

غُرْفَةُ القِياسِ في المَتْجَرِ

More explicitly:

• غرفةُ: nominative, because it is the topic/subject of the sentence
• القياسِ: genitive, because it is the second part of the iḍāfa
• المتجرِ: genitive, because it comes after في

In normal modern writing, these endings are usually not written.

Because it comes after the preposition في, which means in.

Prepositions in Arabic make the following noun genitive.

So:

• في المتجرِ = in the store

This is one of the most basic and important case rules in Arabic.

It could be understood either way, depending on context.

As a full sentence:

• غرفة القياس في المتجر.
• The fitting room is in the store.

As a noun phrase:

• غرفة القياس في المتجر
• the fitting room in the store

If it appears by itself with a period, many learners will read it as a sentence. If it appears on a sign or in a longer expression, it may function more like a phrase.

Literally, قياس is connected with measuring or fitting by size.

So غرفة القياس makes sense as fitting room / measuring room in a store context.

However, another common expression is:

غرفة تبديل الملابس

which is more literally changing room.

So your sentence is understandable and grammatical, but depending on region and context, another phrase may be more common in everyday use.

You would usually remove definiteness from the second noun:

غرفة قياس

That makes the iḍāfa indefinite, so it means:

a fitting room

Compare:

• غرفة قياس = a fitting room
• غرفة القياس = the fitting room

This is a key feature of iḍāfa: the definiteness of the whole phrase depends largely on the second noun.