As @Sylvester pointed out in the newly uploaded Ekottarāgama 8.1 this number appears.
https://suttacentral.net/fr/ea8.1
Translation into English:
… When the Buddha, the Blessed One, resided in the park of Anāthapiṇḍada in Śrāvastī, he said to his bhikṣu: O bhikṣu! No one can be larger than the King of the Asura. He measures 84,000 yojana. His mouth is 1,000 yojana wide. …