How can a woman cover her very long hair with the hijab, and is it permissible for her to cut it for this purpose, and what is the permissible length for cutting?

The length or thickness of the hair does not prevent the lawful hijab (head covering). A woman can let it down and cover it under her abaya. As for cutting a woman's hair, there is no harm in it, as it was reported that the wives of the Prophet, peace and blessings be upon him, used to shorten their hair to lighten their burden, which is evidence of its permissibility for women.