Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
The questioning of the deceased in his grave is a firmly established truth. The deceased sits in his grave, is questioned, and is examined, as stated in authentic texts.
If it is asked: How can he sit in a narrow grave?
The answer is in two ways:
First: Matters concerning the grave belong to the unseen. The believer must affirm what Allah and His Messenger ﷺ have informed us of, without attempting to determine the manner of things whose manner has not been explained to us by Islamic revelation. The conditions of the Barzakh cannot be measured by what we know of worldly life.
Second: The relationship between the soul and the body after death differs from their relationship during life. The soul has conditions whose true nature we cannot comprehend. It is enough to consider that while asleep, a person may see himself traveling, speaking, and meeting living and deceased people, and may experience different places and events, while his physical body remains on his bed and has not moved.
If this is possible during sleep while the body remains as it is, then the relationship between the soul and the body after death is of another nature whose exact manner we cannot comprehend.
Therefore, it is not valid to object to the deceased sitting and being questioned in his grave because of the narrowness or physical form of the grave, for the conditions of the Barzakh differ from those of worldly life.
We must believe in what has been authentically reported from the Prophet ﷺ and say: We hear and obey; we believe. Allah تعالى says:
“But no, by your Lord, they will not truly believe until they make you judge concerning that over which they dispute among themselves, then find no discomfort in their hearts regarding your decision and submit with full submission.” [An-Nisa 4:65]
And Allah knows best.