kore ga saidai no saizu desu.

This is the largest size.

Breakdown of kore ga saidai no saizu desu.

• これ: this
• です: to be
• が: subject particle
• の: possessive case particle

Questions & Answers about kore ga saidai no saizu desu.

Why is it これ and not この?

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.

Why is the particle が used here instead of は?

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.

What does 最大 mean here?