Phòng khách này khá lớn.

In Vietnamese, adjectives can act as the predicate by themselves, so you often do not need a word meaning is.

So:

Phòng khách này khá lớn
literally follows the pattern:

this living room + fairly big

That already means This living room is fairly big.

Using là here would sound unnatural in normal Vietnamese.
Là is mainly used when linking one noun to another, such as:

Anh ấy là giáo viên. = He is a teacher.

But with adjectives, Vietnamese usually just says:

Căn phòng lớn. = The room is big.
Cô ấy đẹp. = She is beautiful.

Literally, phòng means room and khách means guest, so learners often guess guest room.

But phòng khách is a fixed expression meaning living room or sitting room.

So even though the parts look like guest room, the actual meaning is living room.

If you want to say guest bedroom/guest room in the sense of a room for guests to sleep in, Vietnamese would usually say something like:

phòng cho khách = room for guests
phòng ngủ cho khách = guest bedroom

In Vietnamese, demonstratives like this and that usually come after the noun, not before it.

So:

phòng khách này = this living room
phòng khách đó / kia = that living room

This is a very common Vietnamese pattern:

người này = this person
cuốn sách này = this book
cái bàn đó = that table

So the order is natural Vietnamese word order, even though it is the opposite of English.

Khá here means something like fairly, rather, or quite.

So:

khá lớn = fairly big / rather big / quite big

It suggests a noticeable degree, but not an extreme one.

A rough comparison:

hơi lớn = a little big
khá lớn = fairly/quite big
rất lớn = very big

So khá is softer than rất, but stronger than hơi.

Here, khá is functioning as an adverb that modifies the adjective lớn.

So:

khá = fairly / quite
lớn = big

Together:

khá lớn = fairly big

This is a very common pattern in Vietnamese:

rất đẹp = very beautiful
hơi khó = a little difficult
khá xa = fairly far

So in this sentence, khá is not the main idea by itself; it is describing the degree of lớn.

Because degree words like khá, rất, and hơi normally come before the adjective they modify.

So Vietnamese says:

khá lớn = fairly big
rất đẹp = very beautiful
hơi lạnh = a little cold

Putting khá after lớn would sound wrong in standard Vietnamese.

A simple way to remember it is:

degree word + adjective

Yes. You can say:

Căn phòng khách này khá lớn.

That is also correct and natural.

The difference is mostly one of style and explicitness:

Phòng khách này khá lớn. = natural, simple
Căn phòng khách này khá lớn. = a bit more explicit about the room as a physical space

For rooms, căn is a common classifier. In everyday Vietnamese, classifiers are sometimes omitted when the meaning is already clear, as in your sentence.

You may also hear cái phòng khách này, but cái can sound more colloquial, more emphatic, or less neutral depending on context.

Yes, you could say rộng, but the nuance changes a little.

lớn means big/large in a general sense.
rộng means wide/spacious.

So:

Phòng khách này khá lớn. = This living room is fairly big.
Phòng khách này khá rộng. = This living room is fairly spacious.

For a room, rộng often emphasizes the usable space inside.
Lớn is more general and can refer to overall size.

Both are natural, but they are not always exactly identical in nuance.

A rough pronunciation guide is:

Phòng khách này khá lớn

Very roughly for an English speaker:
fawng khak nay kha lurn

But that is only approximate.

A few useful points:

• ph is pronounced like English f
• kh is a breathy sound, somewhat like the ch in Scottish loch or German Bach
• ng at the end of phòng is like the ng in song
• ơ in lớn is a distinct Vietnamese vowel; it does not match English exactly

Tones in this sentence:

• phòng: low/falling tone
• khách: rising tone
• này: low/falling tone
• khá: rising tone
• lớn: rising tone

So a learner should pay special attention not only to the consonants and vowels, but also to the tone pattern.