Is ghusl valid if one enters into a large body of still (not running/moving) water? If yes, then how big does it have to be and how long must one stay under the water?

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

When a person in a state of Janaabah (major ritual impurity) immerses himself in still water to perform Ghusl therein, the Ghusl is valid and sufficient in clearing his liability from the obligation in this regard on condition that the water reaches all parts of his body and its amount is equal to or more than two Qullahs. Ibn Qudaamah, said in Al-Mughni: “If a person in a state of Janaabah, or one whose ablution is invalidated, immerses himself in water whose amount is less than two Qullahs, intending thereby to remove the state of ritual impurity, the water becomes used (not valid to be used for ritual purification after that) and the state of ritual impurity is not removed. [...] If the amount of water is two Qullahs or more, the state of ritual impurity is removed and the water is not affected because it does not carry impurity …” [End of quote]

The volume of two Qullahs is approximately 160.5 liters.

The regulating rule in this regard is the person's certainty or mostly believing that the water has reached his entire body. It is not required to stay in the mentioned water for a specific period of time. It is stated in Ar-Rawdh with its explanatory notes that: “It is sufficient to believe that water has most likely reached the entire body according to the sound opinion in our school of Fiqh. This is supported by the Hadeeth of ‘Aa’ishah : ‘… until when he believed that the water has thoroughly reached his skin...’” [End of quote]

For more benefit on immersing oneself in water for Ghusl, please refer to Fataawa 328663 and 246043.

Allah knows best.