الردهة كبيرة.

A careful MSA pronunciation is approximately:

• ar-rudhah kabīrah when pausing at the end of the sentence
• ar-rudhatu kabīratun in fully inflected, formal reading

A few helpful notes:

• ال is written al-, but here it is pronounced ar-
• كبيرة sounds like ka-BEE-ra(h)
• the final ة often sounds like -a / -ah in pause

Because the next letter is ر, and ر is a sun letter.

In Arabic, when ال comes before a sun letter, the l sound assimilates to the next consonant. So:

• written: الردهة
• pronounced: ar-rudhah, not al-rudhah

This happens in many words, for example:

• الرجل → ar-rajul
• الشمس → ash-shams

So the spelling stays the same, but the pronunciation changes.

Yes. This is a complete Arabic sentence.

Arabic often leaves out the present-tense to be. So:

• الردهة كبيرة literally looks like the lobby big
• but it means The lobby is big

This kind of sentence is very common in Arabic. It is called a nominal sentence.

Its structure here is:

• الردهة = the subject
• كبيرة = the predicate

Because الردهة كبيرة and الردهة الكبيرة mean different things.

• الردهة كبيرة = The lobby is big
• الردهة الكبيرة = the big lobby

In your sentence, كبيرة is not just an adjective attached to the noun. It is the predicate of the sentence, so it stays indefinite.

A very useful contrast:

• بيتٌ كبيرٌ = a big house
• البيت كبيرٌ = the house is big
• البيت الكبير = the big house

Because it agrees with الردهة, which is a feminine singular noun.

In Arabic, adjectives and predicate adjectives usually agree with the noun in:

• gender
• number
• definiteness, when they are directly attributive adjectives

Here, الردهة is feminine singular, so the predicate is also feminine singular:

• masculine: كبير
• feminine: كبيرة

So كبيرة is used, not كبير.

One strong clue is the final ة.

The ending ة is called tāʾ marbūṭa, and it very often marks feminine nouns and adjectives.

So in this sentence:

• الردهة is feminine
• كبيرة is also feminine

Important: ة does not always guarantee femininity in every single word, but it is a very common sign.

Usually:

• at the end of a pause, it sounds like -a or -ah
• when grammatical endings or suffixes are added, it is pronounced as t

So you may hear:

• كبيرة → kabīrah in pause
• كبيرةٌ → kabīratun in full formal pronunciation

Likewise:

• الردهة → ar-rudhah
• الردهةُ → ar-rudhatu

This is a normal pattern with tāʾ marbūṭa.

Because Arabic normally puts descriptive words after the noun, and in a nominal sentence the predicate also usually comes after the subject.

So:

• الردهة كبيرة = subject first, then predicate
• الردهة الكبيرة = noun first, then adjective

This is different from English, where adjectives usually come before the noun:

• English: the big lobby
• Arabic: الردهة الكبيرة

So for an English speaker, the position may feel reversed at first.

The fully vowelled formal form is:

الرُّدْهَةُ كَبِيرَةٌ

In a grammar-focused reading, this shows:

• الردهةُ = nominative subject
• كبيرةٌ = nominative predicate

But in normal Arabic writing, short vowels and case endings are usually omitted. So you usually just see:

الردهة كبيرة

That is completely normal. Learners often study the full endings, but everyday texts usually leave them unwritten.