Why did Allaah swear by the width of the earth, not its length?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

We do not know any text in which Allaah The Almighty swore by the width of the earth. However, it is established that Allaah The Almighty likened the width of Paradise to the width of the heavens and the earth together. The reason for this resemblance – according to Al-Qurtubi – is that Allaah The Almighty used the width instead of the length since length is generally more than width, and if the length is mentioned it does not usually indicate the extent of the width. Az-Zuhri said, "Allaah The Almighty described its (Paradise's) width; as for its length, He alone knows it." Also, the Arabs use width to express the spaciousness of places.

Other scholars said that Paradise was described according to the common usage of the Arabs, i.e. since Paradise is enormously vast and spacious, it is appropriate to liken its size to the width of the heavens and the earth. The verse did not mean to specify the width, but it meant to say that Paradise is larger than whatever you have seen.

Ibn Al-Jawzi said,

“Ibn Qutaybah said, ‘Allaah meant by (Paradise's) width its spaciousness, not the width which is opposite to length. The Arabs use width to describe spacious places, and the Prophet, sallallaahu ‘alayhi wa sallam, addressed the defeated Muslims in the Battle of Uhud saying: ‘The land was too wide for you to flee’ in the same usage. When something widens, it becomes larger; otherwise, it becomes narrow and small.’"

Allaah Knows best.