Я сомневаюсь, что этот размер мне подойдёт.

The dictionary form is сомневаться, which means to doubt.

This verb normally appears with the -ся / -сь ending, because that is simply part of the verb. So:

• я сомневаюсь = I doubt
• ты сомневаешься = you doubt
• он сомневается = he doubts

For English speakers, it may look like a reflexive verb, and historically it is, but in modern Russian you usually just learn сомневаться as the normal verb for to doubt.

So сомневаю by itself is not the standard form here.

Because что introduces a subordinate clause.

The sentence has two parts:

• Я сомневаюсь = I doubt
• что этот размер мне подойдёт = that this size will fit/suit me

In Russian, a subordinate clause introduced by что is normally separated by a comma:

• Я думаю, что...
• Я знаю, что...
• Я сомневаюсь, что...

So the comma is required.

Here что means that.

So:

• Я сомневаюсь, что... = I doubt that...

This is a very common pattern in Russian:

• Я думаю, что... = I think that...
• Я уверен, что... = I’m sure that...
• Я надеюсь, что... = I hope that...

In everyday English, that is often optional, but in Russian что is usually stated explicitly in this structure.

Этот размер is in the nominative case, because it is the subject of the subordinate clause:

• этот размер мне подойдёт = this size will fit me

Here, размер is the thing doing the action of подойдёт.

Breakdown:

• этот = this
• размер = size

Since размер is masculine singular, этот also appears in masculine singular nominative form.

Because the verb подойти in this meaning takes the person in the dative case.

So:

• мне подойдёт = will fit me / will suit me
• literally something like to me it will be suitable

This is very common with Russian verbs that describe suitability, comfort, or happening to someone.

Examples:

• Мне это подходит. = This suits me.
• Тебе идёт это платье. = This dress looks good on you.
• Мне холодно. = I am cold.

So in этот размер мне подойдёт, мне is correct because Russian expresses fit/suit someone with the dative.

Подойдёт is the future form of подойти.

In this sentence, the speaker is talking about whether the size will fit — for example, after trying it on, or in a specific situation that has not yet been confirmed.

So:

• подойдёт = will fit / will suit
• подходит = fits / suits

Compare:

• Я сомневаюсь, что этот размер мне подойдёт.
  = I doubt this size will fit me.

• Этот размер мне не подходит.
  = This size doesn’t fit/suit me.

The future form often sounds natural when you are predicting the result.

Here подойти means to fit or to suit.

This verb has several meanings in Russian depending on context. Some common ones are:

• to approach / come up to
• to suit
• to fit

In clothing or shopping contexts, подойти often means to be the right size / to suit someone:

• Этот размер мне подходит. = This size fits me.
• Этот цвет тебе подойдёт. = This color will suit you.

So even though the verb can literally mean something like approach, here it is idiomatic and means fit/suit.

The difference is mainly future vs. present.

• подходит = fits / is suitable
• подойдёт = will fit / will be suitable

Compare the nuance:

• Этот размер мне подходит.
  This size fits me.
  → sounds like a fact already known

• Этот размер мне подойдёт.
  This size will fit me.
  → sounds like a prediction or expectation

So with Я сомневаюсь..., the future often works well because the speaker is unsure about the outcome.

Yes. Russian word order is flexible, although some orders sound more natural depending on emphasis.

The neutral version is:

• Я сомневаюсь, что этот размер мне подойдёт.

You could also hear:

• Я сомневаюсь, что мне подойдёт этот размер.

This version puts slightly more focus on мне and the idea of whether it will fit me.

In general, Russian word order can shift for emphasis, but the original sentence is a very natural, standard way to say it.

The stress is:

• сомневАюсь
• подойдЁт

So the sentence is pronounced approximately like:

• Я сомневАюсь, что этот размЕр мне подойдЁт.

Stress is important in Russian because it can change pronunciation quite a lot, especially the vowels.

Yes, you could, but it is not exactly the same in tone.

• Я сомневаюсь, что... = I doubt that...
• Я не думаю, что... = I don’t think that...

Both are natural, but сомневаюсь sounds a bit more directly like I have doubts, while не думаю can sound slightly softer or more conversational in some contexts.

So the original sentence is perfectly natural, especially when expressing uncertainty about whether something will fit.