Вроде бы этот размер мне подходит, но, если не он, то я возьму другой.

Вроде бы is a soft, cautious way to say it seems, apparently, or I think so, but I’m not completely sure.

So Вроде бы этот размер мне подходит means something like:

• It seems this size fits me
• I think this size works for me
• This size appears to be right

It adds uncertainty. The speaker is not saying This size definitely fits.

The verb подходить here means to suit, to fit, or to be right for someone. In Russian, the thing that fits is the grammatical subject, and the person it fits takes the dative case.

So:

• этот размер = this size
• мне = to me / for me
• подходит = fits / suits

Literally, the structure is close to:

• This size suits me
• This size is suitable for me

That is why Russian says этот размер мне подходит, not я подхожу.

Because подходить in this meaning normally takes the person in the dative.

Compare:

• Мне подходит этот размер = This size fits me
• Тебе подходит этот цвет = This color suits you
• Нам подходит это время = This time works for us

This is a very common Russian pattern: the thing is the subject, and the person affected is in the dative.

Он refers back to размер.

Since размер is a masculine noun, the pronoun must also be masculine:

• размер → он
• рубашка → она
• платье → оно

So если не он literally means if not it, but in natural English we would usually say:

• if not this one
• if this one doesn’t work
• if not this size

This pattern means if not this one, then... or if this one doesn’t work, then...

It is a common Russian structure:

• если ..., то ... = if ..., then ...

In this sentence:

• если не он = if not this one / if not this size
• то я возьму другой = then I’ll take another one

The не here does not simply mean a basic logical not in English word-for-word style. The whole phrase works idiomatically as if this one isn’t right / if not this one.

In Russian, то often appears in если ..., то ... sentences, just like then in English:

• Если будет дождь, то мы останемся дома.
• If it rains, then we’ll stay home.

In everyday Russian, то is often optional, especially in speech. But here it sounds natural and helps mark the result clearly:

• если не он, то я возьму другой
  = if not this one, then I’ll take another

So то is not absolutely required in every conditional sentence, but it is very common.

Другой means another or a different one.

The noun размер is omitted because it is understood from the context. Russian often does this when the meaning is obvious.

So:

• я возьму другой literally = I’ll take another
• natural English = I’ll take another one / I’ll take a different size

Russian does this a lot with adjectives used on their own when the noun is easy to guess.

Возьму is the perfective future of взять. It means a single completed action: I’ll take.

That fits the context well, because the speaker means a definite decision:

• If this one doesn’t work, I’ll take another one.

If you used буду брать, it would suggest an ongoing or repeated process and would sound unnatural here.

So:

• я возьму другой = I’ll take another one
  not
• I’ll be taking another one

The commas are there because the sentence has more than one clause and also includes an если clause inside a larger sentence.

Structure:

• Вроде бы этот размер мне подходит
• но
• если не он, то я возьму другой

The clause starting with если is a subordinate conditional clause, so it is set off with commas.

That is why you see:

• a comma before но
• a comma after но because the если clause starts there
• a comma after он because the если clause part is separated from the то result clause

So the punctuation is standard for a sentence like:

• ..., but, if not this one, then I’ll take another.

Yes. Подходит can mean more than just physical fit. It can mean:

• fits
• suits
• works for
• is appropriate for

With размер, the most obvious meaning is literal fit, especially for clothes or shoes. But the verb itself is broader.

For example:

• Мне подходит этот вариант = This option works for me
• Тебе подходит эта работа = This job suits you
• Нам подходит пятница = Friday works for us

So here it most likely means this size fits me, but the underlying idea is this size is suitable for me.

The word order here is natural, but Russian word order is fairly flexible.

For example, these are all possible:

• Вроде бы этот размер мне подходит...
• Мне вроде бы подходит этот размер...
• Этот размер вроде бы мне подходит...

They all mean roughly the same thing, but the emphasis changes slightly.

The given version starts with вроде бы, which immediately signals uncertainty, and then introduces этот размер. That sounds very natural in conversation.

Similarly, in the second half:

• то я возьму другой
• то другой возьму

Both are possible, but то я возьму другой is the most neutral and straightforward.