kono syatu ha saizu ga tyoudo ii node, watasi ni aimasu.

They are doing different jobs.

• このシャツは sets this shirt as the main topic.
• サイズがちょうどいい says something specific about that shirt: its size is just right.

So the structure is roughly:

• As for this shirt, its size is just right, so it fits me.

A very common Japanese pattern is:

• A は B が ...
• As for A, B is ...

Here:

• A = このシャツ
• B = サイズ

So は marks the overall topic, while が marks the thing being described inside that topic.

ちょうどいい means just right.

In this sentence, it means the size is neither too big nor too small. It has a very natural, everyday feeling.

Examples:

• サイズがちょうどいい = The size is just right.
• 量がちょうどいい = The amount is just right.
• タイミングがちょうどいい = The timing is perfect/just right.

So here it specifically refers to the shirt’s size being a good fit.

Both ので and から can mean because, but they have different tones.

• ので sounds a little softer, more explanatory, and slightly more formal/polite.
• から sounds more direct and conversational.

So:

• サイズがちょうどいいので、私に合います。 = Because the size is just right, it fits me.

If you used から:

• サイズがちょうどいいから、私に合います。

That is also grammatical, but ので sounds a bit gentler and more natural in careful speech.

Because the verb 合う in this meaning does not take を for the person.

Here 合う means to fit, to suit, or to match someone. The person who something fits is marked with に.

So:

• 私に合います = It fits me / It suits me

Think of it as:

• This shirt fits to me or is suitable for me Even though that is not natural English, it helps explain why Japanese uses に.

You will often see this pattern:

• その服は彼に合う。 = Those clothes suit him.
• この色はあなたに合います。 = This color suits you.

In this sentence, 合います means fits me or suits me.

With clothing, 合う can mean:

• the size fits
• it suits the person
• it is appropriate for them

Because the sentence already says サイズがちょうどいい, the main idea here is that the shirt fits properly in size.

So the nuance is very close to:

• The size is just right, so it fits me.

But 合う is a little broader than just can I physically wear it?

For example:

• 着られます = I can wear it
• 入ります = It goes on / fits in a physical sense
• 合います = It fits me / suits me

いい is the normal everyday form of よい.

They mean the same thing:

• よい = good
• いい = good

In modern speech, いい is much more common in ordinary conversation. So ちょうどいい is the standard natural expression.

One thing to remember is that いい behaves irregularly in some forms because it comes from よい:

• よくない = not good
• よかった = was good

So although the dictionary/base idea is related to よい, learners will hear いい constantly.

サイズがちょうどいい is the most natural way to say the size is just right in this sentence.

Using が here focuses on size as the thing that has the quality just right.

If you said サイズは, it would sound more contrastive, like:

• As for the size, it’s fine...
  possibly implying that something else is not fine, such as the color or design.

So:

• サイズがちょうどいい = The size is just right.
• サイズはちょうどいい = The size is just right, at least. / As for the size, it’s fine.

That is why が is the more neutral choice here.

Yes, that is possible, but the meaning becomes less specific.

• このシャツはちょうどいい could mean:
  • this shirt is just right in size
  • this shirt is just right overall
  • this shirt is ideal in some broader sense

By adding サイズが, the speaker makes it clear that they are specifically talking about the size.

So:

• このシャツはサイズがちょうどいいので、私に合います。 clearly means the fit in terms of size is right.

In normal Japanese writing, you would usually write it without spaces:

• このシャツはサイズがちょうどいいので、私に合います。

The spaces in your example are there only to make it easier for learners to see the parts of the sentence.

Japanese normally does not separate words with spaces the way English does.

So:

• このシャツ = normal writing
• この　シャツ = learner-friendly spacing

The meaning is exactly the same.

この must be followed by a noun, while これ stands alone.

So:

• このシャツ = this shirt
• これ = this

You cannot say これシャツ.

In this sentence, the speaker is directly modifying シャツ, so この is required.

Compare:

• このシャツは… = This shirt...
• これはシャツです。 = This is a shirt.

Yes. The sentence is polite overall because the main verb is in the ます form:

• 合います

Japanese often mixes plain forms inside a sentence with a polite ending at the end. That is completely normal.

Before ので, adjectives and verbs usually appear in their normal non-ます form:

• ちょうどいいので not
• ちょうどいいですので in ordinary standard usage

So this sentence is naturally polite:

• plain adjective phrase: サイズがちょうどいい
• polite final verb: 合います

Yes, very often.

Japanese frequently omits things that are understood from context. If it is already obvious that the speaker means me, the sentence could be:

• このシャツはサイズがちょうどいいので、合います。

However, keeping 私に makes the meaning extra clear:

• it fits me
• not necessarily someone else

So including 私に is perfectly natural when the speaker wants to be explicit.

A casual version would often be:

• このシャツ、サイズがちょうどいいから、私に合う。

Changes:

• は may be dropped after このシャツ
• ので becomes から
• 合います becomes 合う

This sounds more conversational and less formal.

Polite:

• このシャツはサイズがちょうどいいので、私に合います。

Casual:

• このシャツ、サイズがちょうどいいから、私に合う。

Both mean basically the same thing.