Lá thư này khá dài.

A natural breakdown is:

• lá = a classifier used with some flat, thin items, and also commonly used in lá thư for letter
• thư = letter, mail
• này = this
• khá = quite, fairly, rather
• dài = long

So the structure is basically:

lá thư này + khá dài
= this letter + is quite long

Vietnamese often does not use a verb like English to be when describing something with an adjective.

So:

• Lá thư này khá dài = This letter is quite long
• literally closer to: This letter quite long

This is very normal in Vietnamese. Adjectives can function directly as the predicate.

In Vietnamese, demonstratives like this and that usually come after the noun.

So:

• lá thư này = this letter
• lá thư đó = that letter

This is the opposite of English word order.
A useful pattern is:

noun + này/đó/kia
= this/that + noun

Yes, lá can mean leaf, but here it is functioning as a classifier.

Vietnamese often uses classifiers with nouns. In lá thư, lá is the conventional classifier for thư in many contexts.

So:

• lá = classifier
• thư = letter

Together, lá thư means a letter.

This is something English speakers often find strange at first, because English usually does not need this kind of classifier in ordinary noun phrases.

Yes, you may hear thư này khá dài, and it can sound natural depending on context and style. But lá thư này khá dài is a very standard, complete form.

In general:

• lá thư sounds a bit fuller and more traditional/standard
• thư alone can also be used, especially in everyday speech or when the classifier is not important

So both can occur, but the version with lá is very good Vietnamese.

Khá means quite, fairly, or rather. It gives a moderate degree.

So:

• dài = long
• khá dài = quite long / fairly long

It is weaker than rất in many cases:

• dài = long
• khá dài = fairly/quite long
• rất dài = very long

It is usually stronger than hơi:

• hơi dài = a little long / somewhat long
• khá dài = fairly long

It can do both, depending on context.

• For physical objects, dài means long
• For things like speeches, books, letters, meetings, etc., it can mean long in the sense of lengthy

So in Lá thư này khá dài, dài means the letter is lengthy in content.

The pattern is:

[noun phrase] + [degree word] + [adjective]

So here:

• Lá thư này = noun phrase
• khá = degree word
• dài = adjective

A very useful model is:

• Cuốn sách này rất hay = This book is very interesting/good
• Căn phòng này hơi nhỏ = This room is a little small
• Lá thư này khá dài = This letter is quite long

You can put không before the adjective phrase:

• Lá thư này không dài = This letter is not long
• Lá thư này không khá dài is not the usual way to say it

If you want to negate the whole idea naturally, the common form is:

• Lá thư này không dài lắm = This letter is not very long
• Lá thư này không quá dài = This letter is not too long

So usually không goes before the adjective, not before just khá.

A very common way is to add phải không? or use a yes-no question pattern.

For example:

• Lá thư này khá dài, phải không? = This letter is quite long, right?
• Lá thư này có khá dài không? = Is this letter quite long?

In everyday Vietnamese, the first type is often especially natural when you are checking something the listener is likely to agree with.

A rough pronunciation guide is:

• Lá — rising tone
• thư — level vowel sound somewhat like thư, with no exact English equivalent
• này — rising tone
• khá — rising tone
• dài — falling-rising tone in the South, slightly different in the North, depending on accent

A rough English-style approximation might be:

lah thuh nye khah yai

But that is only approximate. The tones matter a lot in Vietnamese, especially on words like lá, này, khá, and dài.

Yes, it is completely natural. It is simple, grammatical, and useful.

It sounds like something you could say when commenting on a letter you are reading. It is also a good example sentence because it shows several important Vietnamese features at once:

• a classifier: lá
• noun + demonstrative: thư này
• no verb for is
• a degree word: khá
• an adjective used as predicate: dài