kore ga saidai no saizu desu.

これ means this and can stand by itself as a noun: this one / this thing.

• これが最大のサイズです。 = This is the largest size.

この cannot stand alone. It must come before a noun.

• このサイズ = this size
• このシャツ = this shirt

So in this sentence, これ is correct because the speaker is saying this one.

In this sentence, が marks これ as the thing being identified.

• これが最大のサイズです。 = This is the largest size.

Using が often gives a sense of this is the one or this is the one that fits that description.

If you said これは最大のサイズです, that is also possible, but the nuance is a little different:

• これが: focuses on which one it is
• これは: makes this the topic, like as for this, it is the largest size

So が is very natural if someone is pointing to an item and identifying it.

最大 is read さいだい and means maximum or largest.

It is a somewhat formal or objective word often used in contexts like:

• 最大の問題 = the biggest problem
• 最大の理由 = the main/largest reason
• 最大サイズ = maximum size / largest size

In this sentence, 最大 means the biggest available size.

Here, 最大 is being used like a noun, and の connects it to another noun.

• 最大のサイズ = literally something like the size of maximum
• natural English: the largest size / the maximum size

This is a very common pattern in Japanese:

• 日本の会社 = a Japanese company
• 学生の本 = a student's book
• 最大のサイズ = the largest size

So の is the linker between the two nouns.

Yes, 最大サイズ is also common.

Both are natural, but there is a slight difference in feel:

• 最大のサイズ sounds a little more like a full descriptive phrase
• 最大サイズ sounds more like a compound term or label

For example, on product packaging or a chart, 最大サイズ is very common. In a full sentence, 最大のサイズ sounds a bit more explanatory.

です is the polite copula, roughly like is / am / are in this kind of sentence.

So:

• これが最大のサイズです。 = polite
• これが最大のサイズだ。 = plain
• これが最大のサイズ。 = casual, with the copula effectively dropped in conversation

It makes the sentence polite and complete.

Yes. サイズ is a very common Japanese loanword from English size.

It is especially common for things like:

• clothing sizes
• shoe sizes
• product dimensions/options

Japanese also has words like 大きさ, which means size / bigness, but サイズ is often more natural when talking about selectable sizes in shopping or products.

So in a store, サイズ is exactly what you would expect to hear.

The structure is:

• これ = this
• が = subject marker
• 最大のサイズ = the largest size
• です = is (polite)

So the sentence is literally something like:

• This + is-the-one-that-is + the largest size + is

A more natural English rendering is:

• This is the largest size.

Japanese often puts the main predicate at the end, so the word order feels different from English.

Yes, often it can.

For example:

• これが一番大きいサイズです。

This also means This is the largest size.

The difference is mostly style:

• 最大 sounds a bit more formal, compact, or technical
• 一番大きい sounds more everyday and descriptive

Both are natural, but 最大 is very common in product, business, and informational contexts.

It is read さいだい.

• 最 = さい
• 大 = だい

So the full sentence is read:

これ が さいだい の サイズ です。

Because これ already means this one / this thing.

Japanese has two related patterns:

• これ = this one
• この + noun = this + noun

So:

• これが最大のサイズです。 = This is the largest size.
• このシャツが最大のサイズです。 = This shirt is the largest size.

Since これ already works as a complete noun, nothing needs to come after it.