المدرجات كبيرة.

In Modern Standard Arabic, the verb to be is usually not expressed in the present tense.

So:

• المدرجات كبيرة = The stands / amphitheaters are large
• literally, it is more like The stands large

This kind of sentence is called a nominal sentence.
If you wanted a past or future meaning, Arabic would use an actual verb:

• كانت المدرجات كبيرة = The stands were large
• ستكون المدرجات كبيرة = The stands will be large

It describes المدرجات, so in meaning it works like large in English. But grammatically, in this sentence it is the predicate of a nominal sentence, not just an adjective inside a noun phrase.

Arabic normally places describing words after the noun they describe.

Compare:

• المدرجات كبيرة = The stands are large
  
  • a complete sentence
• المدرجات الكبيرة = the large stands
  
  • just a noun phrase, not a full sentence by itself

So yes, كبيرة expresses a quality, but in this sentence it is functioning as the khabar (خبر), the predicate.

Because المدرجات كبيرة is a full sentence, while المدرجات الكبيرة is only a noun phrase.

• المدرجات كبيرة = The stands are large
• المدرجات الكبيرة = the large stands

In Arabic, when a word is a normal adjective directly modifying a noun, it agrees with that noun in definiteness too. So if the noun has الـ, the adjective also has الـ:

• المدرجات الكبيرة = the large stands

But in your sentence, كبيرة is not an attributive adjective inside the noun phrase. It is the predicate, so it is normally indefinite.

This is one of the most important grammar points in Arabic:

Non-human plurals are usually treated as feminine singular for agreement in MSA.

So even though المدرجات is plural, it is a non-human plural noun. Because of that, the predicate/adjective appears in the feminine singular form:

• المدرجات كبيرة

not:

• المدرجات كبيرات

The form كبيرات would be used mainly with plural feminine humans.

So the rule is:

• human plural → usually plural agreement
• non-human plural → usually feminine singular agreement

That is why كبيرة is exactly what you expect here.

It is a plural form ending in ـات, and that ending often makes learners think of regular feminine plurals. But here the key point is not just the ending — it is the fact that the noun is a non-human plural.

Its singular is typically مُدَرَّج. Even though the singular is not feminine, the plural المدرجات still takes feminine singular agreement because it is a non-human plural noun.

So for agreement purposes, you should think:

• المدرجات = plural non-human noun
• therefore → adjective/predicate often becomes feminine singular

The singular is مُدَرَّج.

So:

• مُدَرَّج = one stand / one amphitheater / one tiered hall, depending on context
• مُدَرَّجات = plural

This is useful because learners often see ـات and assume the singular must end in ـة, but that is not always true. Some non-human nouns form their plural with ـات even when the singular does not end in ـة.

The fully vowelled form is:

الْمُدَرَّجَاتُ كَبِيرَةٌ

With case endings:

• المدرجاتُ = nominative, because it is the subject/topic of the nominal sentence
• كبيرةٌ = nominative, because it is the predicate

In normal unvowelled writing, those endings are usually not written, so you simply see:

المدرجات كبيرة

A careful MSA pronunciation with full endings is:

al-mudarrajātu kabīratun

In normal pausal pronunciation, you will often hear something closer to:

al-mudarrajāt kabīra

A few helpful notes:

• الـ in المدرجات is pronounced al- here, because م is a moon letter
• ج in MSA is usually pronounced like j in judge
• the final ة in كبيرة is pronounced like -a when pausing

Not for the same simple meaning.

The normal, straightforward sentence is:

• المدرجات كبيرة

If you put كبيرة first, you are no longer making the same basic nominal sentence in the usual way. Arabic does allow other word orders in certain structures, but for a beginner, the safe form for The stands are large is:

• المدرجات كبيرة

So if your goal is to say this exact meaning clearly and correctly, keep the order as it is.