Этот размер мне не подходит, потому что туфли слишком тесные.

Because размер is a masculine singular noun, and here it is in the nominative case as the subject of the sentence.

So:

• этот = this (masculine singular)
• размер = size

They must agree:

• этот размер
• not эта размер or это размер

The verb подходить in this meaning works like to suit / to fit someone.

In Russian, the thing that fits is the subject, and the person is put in the dative case:

• Этот размер мне подходит = This size fits me / suits me
• literally: This size is suitable to me

So:

• мне = to me
• подходит = fits / suits

English uses me as a direct object, but Russian uses the dative here.

Because подходить takes the person in the dative case, not the accusative.

Compare:

• мне = to me (dative)
• меня = me (accusative/genitive, depending on use)

With подходить:

• Это мне подходит = This suits me
• not Это меня подходит

This is just the normal pattern of the verb.

Because the subject is размер, which is singular.

The verb agrees with размер:

• Этот размер подходит = singular
• Эти размеры подходят = plural

Even though мне is also in the sentence, the verb does not agree with мне. It agrees with the subject размер.

Here подходить means to fit, to suit, or to be right for.

It is a very common Russian verb with several related meanings:

• Этот размер мне подходит = This size fits me
• Это платье тебе подходит = This dress suits you
• Завтра мне не подходит = Tomorrow doesn’t work for me

So the core idea is being appropriate or suitable.

Because потому что introduces a subordinate clause, and in Russian that clause is separated by a comma.

So:

• Этот размер мне не подходит, потому что туфли слишком тесные.

This is standard Russian punctuation, just like English often uses a comma before because when separating clauses.

Потому что means because.

Yes, in this use it is written as two words:

• потому что

It is a fixed conjunction used to give a reason:

• Я дома, потому что устал. = I’m at home because I’m tired.

In everyday Russian, туфли is normally used in the plural because shoes come as a pair and the word is most often talked about that way.

So:

• туфли = shoes

That is why the adjective is also plural:

• туфли слишком тесные

You may see туфля as a singular form meaning a shoe, but in normal shopping or everyday speech, туфли is much more common.

Because тесные agrees with туфли.

• туфли is plural
• so the adjective must also be plural
• тесные = tight

Compare:

• туфли тесные = the shoes are tight
• ботинок тесный = the shoe/boot is tight (masculine singular)
• рубашка тесная = the shirt is tight (feminine singular)

Russian adjectives change form to match the noun in gender, number, and case.

Because тесные is an adjective describing the noun туфли.

• туфли слишком тесные = the shoes are too tight

But тесно is an adverb or a predicative form, used differently:

• Мне тесно в этих туфлях. = These shoes are tight on me / I feel cramped in these shoes.

So both are possible in Russian, but they are different structures:

• туфли тесные = adjective + noun
• мне тесно = predicative expression about how it feels

Слишком means too or excessively.

It usually goes before the adjective or adverb it modifies:

• слишком тесные = too tight
• слишком большой = too big
• слишком быстро = too fast

So in this sentence:

• туфли слишком тесные = the shoes are too tight

Yes. That is completely natural.

Russian word order is flexible, so both are fine:

• Этот размер мне не подходит
• Мне не подходит этот размер

The first version starts with this size, so it sounds a bit more neutral or topic-focused on the size itself.

The second version starts with to me / for me, which can give slightly more emphasis to your personal situation.

Both are correct.

Yes, but the meaning shifts slightly.

• Этот размер мне не подходит focuses on the size
• Эти туфли мне не подходят focuses on the shoes

Both are natural, and both could make sense in a store. Your original sentence is especially useful when talking specifically about the size you tried on.

No. Подходить is used very broadly in Russian.

It can mean:

• to fit physically
  
  • Этот размер мне подходит.
• to suit someone’s appearance or style
  
  • Этот цвет тебе подходит.
• to be convenient / acceptable
  
  • Пятница вам подходит? = Does Friday work for you?
• to match requirements
  
  • Этот вариант нам не подходит.

So it is a very useful everyday verb.

Yes, very natural.

A Russian speaker could easily say this when trying on shoes or clothes:

• Этот размер мне не подходит, потому что туфли слишком тесные.

You might also hear shorter, more conversational versions such as:

• Этот размер мне не подходит — туфли слишком тесные.
• Мне нужен размер побольше. = I need a bigger size.
• Эти туфли мне тесны. = These shoes are tight for me.

But your sentence is fully correct and natural.