Не могли бы Вы сказать, какая длина у этой полки и поместится ли она между окном и дверью?

It literally contains не + могли бы, so word-for-word it looks like Could you not say..., but in real Russian this is a very common polite formula meaning:

• Could you tell me...
• Would you be able to say...
• Would you mind telling me...

So Не могли бы Вы сказать... is a soft, polite request, not a negative complaint.

A few similar polite openings are:

• Не могли бы Вы помочь? — Could you help?
• Не могли бы Вы повторить? — Could you repeat that?

This use of не makes the request more polite, not truly negative.

Capital Вы is a polite written form when addressing one person respectfully, similar to formal you.

So here:

• Вы = polite/formal singular you
• вы can also mean plural you, or sometimes informal lowercase polite usage in less formal writing

In modern Russian, capitalizing Вы is common in formal letters, business communication, customer service, and other respectful contexts. In everyday writing, many people just write вы.

Могли бы is a standard polite conditional form.

Compare:

• Вы можете сказать...? — Can you tell me...?
  Polite enough, but more direct.
• Не могли бы Вы сказать...? — Could you tell me...?
  Softer and more polite.

Grammatically:

• могли is past tense plural/formal
• бы creates a conditional meaning, roughly would/could

So не могли бы Вы сказать is basically would you be able to tell me.

Russian often asks about dimensions with a noun:

• какая длина — what is the length
• какая ширина — what is the width
• какая высота — what is the height

So какая длина у этой полки? literally means what is the length of this shelf?

This is very natural in Russian, especially in shops, measurements, furniture descriptions, and formal questions.

You could also hear other ways in conversation, but this pattern is extremely common:

• Какая длина у стола?
• Какая ширина у шкафа?

Полки is genitive singular of полка.

The structure у + genitive often means at, by, or in this kind of sentence, of/for in the sense of possession or characteristic:

• у этой полки = this shelf has / of this shelf
• so какая длина у этой полки = what is the length of this shelf

This is a very common Russian pattern:

• У машины хорошая скорость. — The car has good speed.
• У комнаты два окна. — The room has two windows.
• Какая высота у потолка? — What is the height of the ceiling?

Поместится comes from поместиться, which means:

• to fit
• to have enough room
• to be accommodated in a space

So поместится ли она между окном и дверью? means:

• Will it fit between the window and the door?

This verb is often used for objects fitting into spaces:

• Стол поместится в комнате? — Will the table fit in the room?
• Все вещи поместятся в машину? — Will all the things fit in the car?

Ли is a particle used to form an indirect yes/no question.

Compare:

• direct question: Она поместится? — Will it fit?
• indirect question: ...сказать, поместится ли она... — ...tell me whether it will fit...

So here the whole sentence contains embedded questions after сказать:

• какая длина у этой полки
• поместится ли она между окном и дверью

A useful rule: ли usually comes right after the word being questioned, often the verb.

In Russian, ли normally follows the element it applies to.

Here the key thing being questioned is the verb поместится:

• поместится ли она = whether it will fit

This is the most neutral and natural order.

Other orders can exist, but they change emphasis or sound less neutral:

• она ли поместится would mean something like is it the one that will fit?, which is a different emphasis.

So for ordinary whether... clauses, verb + ли + subject is very common.

Она refers to эта полка — this shelf.

Since полка is a feminine noun, Russian uses the feminine pronoun:

• полка → она

So:

• какая длина у этой полки — what is the length of this shelf
• поместится ли она... — whether it will fit...

In English, a learner might expect it, but in Russian pronouns agree with the noun’s grammatical gender.

After между meaning between, Russian normally uses the instrumental case.

So:

• окно → окном
• дверь → дверью

That gives:

• между окном и дверью — between the window and the door

This is a very common pattern:

• между столом и стеной — between the table and the wall
• между домами — between the houses

It is one polite request containing two embedded questions.

The speaker is asking someone to tell them two things:

1. какая длина у этой полки — what the shelf’s length is
2. поместится ли она между окном и дверью — whether it will fit between the window and the door

So the whole sentence is one larger request:

• Could you tell me X and Y?

Yes, but repeating the noun would sound heavier and less natural.

Compare:

• ...какая длина у этой полки и поместится ли она... — natural
• ...какая длина у этой полки и поместится ли эта полка... — grammatical, but repetitive

Russian, like English, usually uses a pronoun once the noun is already clear.

Сказать is the normal verb for to say / to tell in a context like tell me the information.

• сказать = to say / tell
• рассказать = to tell in detail / narrate
• говорить = to speak / say in a more general or ongoing sense

So:

• Не могли бы Вы сказать... = Could you tell me...
  This is exactly the right choice.

Using рассказать would sound odd because the speaker is not asking for a story or long explanation. They just want specific information.

It sounds natural, polite, and fairly formal. It would fit situations like:

• asking a salesperson about furniture
• writing to a shop or seller
• speaking politely to someone you do not know

A slightly less formal version might be:

• Не могли бы вы сказать, какая длина у этой полки и поместится ли она между окном и дверью?

And a more conversational version could be:

• Скажите, пожалуйста, какая у этой полки длина и поместится ли она между окном и дверью?

All of these are natural; the original is just especially polite.