مساحة التخزين صغيرة.

A good Egyptian-style pronunciation is:

mesāḥet et-takhzīn soġayyara

A few useful notes:

• مساحة by itself is roughly mesāḥa.
• In مساحة التخزين, the ة is heard as -et because it is followed by another noun in an iḍāfa construction, so مساحة becomes mesāḥet.
• In التخزين, the ال is pronounced et-, not el-, because ت is a sun letter, so the l sound assimilates.
• صغيرة in Egyptian is often closer to soġayyara than the more formal ṣaghīra.

Because Arabic usually leaves out the verb to be in the present tense.

So:

• مساحة التخزين صغيرة = The storage space is small

Literally, it is more like:

• storage space small

This is completely normal in Arabic. If you wanted was or will be, then Arabic would use a verb.

Because مساحة التخزين is an iḍāfa construction, which is very common in Arabic.

Here it works like this:

• مساحة = space
• التخزين = the storage / storing

Together:

• مساحة التخزين = the storage space
• literally: space of the storage

In an iḍāfa:

• the first noun usually does not take الـ
• the whole phrase becomes definite if the second noun is definite

So مساحة التخزين is definite even though مساحة itself does not have الـ.

Because the noun being described is مساحة, and مساحة is a feminine noun.

In Arabic, adjectives agree with the noun they describe in gender and number.

So:

• مساحة = feminine
• therefore small must be feminine too
• صغيرة = small (feminine)

It does not agree with التخزين here, because التخزين is not the main noun of the phrase. The main noun is مساحة.

Because here صغيرة is the predicate of the sentence, not just an adjective inside the noun phrase.

Compare these:

• مساحة التخزين صغيرة = The storage space is small
• مساحة التخزين الصغيرة = the small storage space

So:

• صغيرة without الـ = it is telling you something about the subject
• الصغيرة with الـ = it is directly modifying the noun as part of one noun phrase

This is a very common point for learners.

It is understandable and natural enough, especially in a technical or phone/computer context. It does sound a bit neutral/formal, though, because التخزين is somewhat technical.

In everyday Egyptian, people might also say things like:

• سعة التخزين صغيرة = the storage capacity is small
• مساحة تخزين الموبايل صغيرة = the phone’s storage space is small
• ذاكرة الموبايل صغيرة = the phone’s memory is small

So your sentence is good, but in casual speech people may choose slightly different wording depending on the device and context.

They are related, but not exactly the same.

• مساحة = space
• سعة = capacity

So:

• مساحة التخزين focuses on storage space
• سعة التخزين focuses more on storage capacity

In tech contexts, both can appear, but سعة التخزين often sounds a bit more like specifications, while مساحة التخزين is very natural for the idea of available or total storage space.

Not in normal, neutral speech.

The usual order here is:

• subject
  • predicate
• مساحة التخزين
  • صغيرة

So the normal sentence is:

• مساحة التخزين صغيرة

Starting with صغيرة would sound marked, poetic, or unnatural in ordinary conversation. For a learner, it is best to stick with the standard order above.