tenin ha teinei de, watasi ni niau iro to tyoudo ii saizu no uwagi wo sugu mitukete kureta.

It has two main parts:

• 店員は丁寧で = The clerk was polite / and polite
• 私に似合う色とちょうどいいサイズの上着をすぐ見つけてくれた = (they) quickly found me a jacket in a color that suited me and in a size that was just right

So the sentence is basically:

As for the clerk, they were polite, and they quickly found me a jacket with a suitable color and size.

The second part is the main action, and the first part gives extra description about the clerk.

は marks 店員 as the topic: as for the clerk...

It does not necessarily mean strong contrast here. It simply sets up who the sentence is about. After that, Japanese can keep talking about the same person without repeating them.

So:

• 店員は = As for the clerk...
• later 見つけてくれた = [the clerk] found ... for me

The subject of the later verb is omitted because it is already understood from the topic.

丁寧 is a な-adjective.

A な-adjective changes form depending on how it is used:

• before a noun: 丁寧な店員 = a polite clerk
• as a predicate: 店員は丁寧だ = The clerk is polite
• connecting to another clause: 丁寧で = being polite / and polite

So in this sentence, 丁寧で is the connecting form. It links polite to the next action:

• 店員は丁寧で、... = The clerk was polite, and...

You do not use 丁寧な here because it is not directly modifying a noun.

Because 丁寧で is being used as a connector, and Japanese often lets the final verb set the time frame for the whole sentence.

The sentence ends with くれた, which is past tense, so the whole event is understood as past. That makes 店員は丁寧で naturally understood as describing the clerk’s politeness in that past situation.

So even though 丁寧で itself is not explicitly marked as past, the full sentence still reads as past.

It is still the clerk.

Japanese often omits subjects when they are already clear. Since 店員は appears at the start, the later verb is understood to have the same subject unless something new is introduced.

So the sentence does not need to repeat 店員が or 店員は again.

The verb 似合う does not work like a direct-object verb in English.

With 似合う, the person or thing that something suits is marked with に:

• この色は私に似合う = This color suits me
• literally: As for this color, it suits to me

So:

• 私に = to me / on me
• 似合う = suits

That is why に is used, not を.

Japanese does not need a relative pronoun like that, which, or who here.

Instead, the modifying clause goes directly before the noun:

• 私に似合う色
• literally: me-suits color
• natural English: a color that suits me

This is a very common Japanese pattern:

• 昨日買った本 = the book I bought yesterday
• 日本に住んでいる人 = a person who lives in Japan

So 私に似合う is just a clause modifying 色.

Because 色 is the noun being modified from the outside.

If you made it a full sentence, you could say:

• その色が私に似合う = That color suits me

But in a relative clause, Japanese often leaves the noun outside the clause:

• 私に似合う色 = the color that suits me

So 色 is understood as the thing that would be the subject of 似合う, but it appears after the clause instead of inside it.

Here, 色 and サイズ are being grouped together before 上着:

• [私に似合う色 と ちょうどいいサイズ] の 上着

This means something like:

• a jacket with a color that suits me and a size that is just right

Japanese can use one の after a coordinated pair like AとB to connect the whole group to the noun that follows.

So the single の applies to both:

• 私に似合う色
• ちょうどいいサイズ

Both describe the 上着.

ちょうどいい is a very common expression meaning:

• just right
• exactly right
• perfect

So:

• ちょうどいいサイズ = the right size / a size that fits just right

It is made from:

• ちょうど = exactly
• いい = good

But together they function as a set phrase.

Because 上着 is the direct object of 見つけてくれた.

The clerk found the jacket, not the color by itself or the size by itself.

Everything before 上着 is just describing what kind of jacket it was:

• 私に似合う色
• ちょうどいいサイズ
• の上着

Then the whole noun phrase gets を:

• 私に似合う色とちょうどいいサイズの上着を
• the jacket with a suitable color and the right size
  • object marker

すぐ modifies the verb phrase 見つけてくれた.

It means:

• quickly
• right away
• without delay

So the sense is that the clerk did not take long to find the jacket.

くれた adds a benefactive nuance: someone did something for me or to my benefit.

So:

• 見つけた = found
• 見つけてくれた = found for me / kindly found for me

This is a very common Japanese pattern:

• Vてくれる = someone does V for the speaker or the speaker’s side

Here it shows that the clerk’s action helped the speaker. It has a grateful or appreciative nuance.

Because Japanese normally does not have articles like a, an, or the.

So 上着 by itself can mean:

• a jacket
• the jacket
• sometimes even jackets

The exact meaning depends on context. In this sentence, English naturally uses a jacket, because the clerk found one for the speaker.