Между диваном и окном осталось всего двадцать сантиметров, и проход получился слишком тесным.

Because the preposition между normally takes the instrumental case in modern Russian when it means between.

So:

• диван → диваном
• окно → окном

That is why you get между диваном и окном = between the sofa and the window.

A learner can think of this as a fixed pattern:

• между столом и стулом
• между домами
• между мной и тобой

Here всего means only, just, or a mere.

So осталось всего двадцать сантиметров means:

• only twenty centimeters were left
• there were just twenty centimeters left

It adds the idea that the amount is surprisingly small.

Be careful: всего can mean different things in other contexts, but here it does not mean everything.

Russian often uses neuter singular past tense with quantities, measurements, time spans, and similar amount-expressions.

So:

• осталось двадцать сантиметров
• осталось три метра
• прошло два года
• осталось пять минут

Even though двадцать сантиметров is plural in meaning, the verb is very often treated as referring to the amount as a whole.

Using остались is not impossible in all situations, but here осталось is the most natural choice.

Because after numerals like 5, 6, 7... 20, 30, 40 and so on, the noun usually goes in the genitive plural.

So:

• 1 сантиметр
• 2 сантиметра
• 5 сантиметров
• 20 сантиметров

Since двадцать belongs to the type that takes genitive plural, the correct form is сантиметров.

Yes. In Russian, остаться is very natural for talking about space left, time left, money left, and so on.

Examples:

• Осталось десять минут. = Ten minutes are left.
• У меня осталось мало денег. = I have little money left.
• Здесь осталось мало места. = There is little space left here.

So между диваном и окном осталось всего двадцать сантиметров is a completely normal way to say that only twenty centimeters of space were left.

Получиться here means to turn out, to end up, or to come out in a certain way.

So:

• проход получился слишком тесным = the passage turned out too narrow/cramped

This is a little different from simply saying:

• проход был слишком тесным = the passage was too narrow

Using получился emphasizes the result of how things were arranged. It suggests: after putting the sofa and the window in that relation, the passage ended up too narrow.

Because after verbs like получиться, стать, and оказаться, the adjective describing the result is usually put in the instrumental case.

So:

• проход получился тесным
• он стал сильнее
• она оказалась правой
• дом оказался слишком маленьким

Here тесным is the instrumental form of тесный.

This is a very common Russian pattern with verbs meaning become, turn out, or prove to be.

Here проход means the passageway, walkway, or gap you can pass through.

In this sentence, it refers to the space between the sofa and the window where a person would walk.

So it is not an abstract passage in a text, and not a pass or permit. It is the physical route through a space.

Слишком means too, while очень means very.

So:

• очень тесный = very narrow/cramped
• слишком тесный = too narrow/cramped

Слишком implies that the narrowness is a problem or is more than acceptable.

That fits this sentence well: the passage was not just narrow, it was unacceptably narrow.

Yes. Russian word order is flexible, and different versions can shift emphasis.

For example:

• Между диваном и окном осталось всего двадцать сантиметров...
• Всего двадцать сантиметров осталось между диваном и окном...
• Осталось всего двадцать сантиметров между диваном и окном...

All of these are possible, but the original sounds natural and neutral. It first sets the location between the sofa and the window, then states how little space was left.

Russian often uses word order to control focus and emphasis, not just grammar.