بسم الله الرحمن الرحيم
Yes, it is lawful. No particular length of amamah is proved from Sahih Hadith. (See: Khasail Nabwi, Jam’ al-Wasail etc). However the length of amamah should not exceed from the length that is usually worn by Ulama and Mashaikh.
(2) It is proved that the Messenger of Allah (may peace and blessings of Allah be upon him) wore amamah of black, white and other colour. Since amamah is also like cloth and each type of cloth is lawful for males except saffron colour, though white cloth is better. However, pure red colour is somehow makrooh. Thus according to Shariah each colour of amamah is lawful other than saffron colour, no colour is prohibited until there is no special reason (resemblance with any particular sect). (See: Fath al-Bari, 4/61, pub. Darul Marifah, Shamail Tirmidhi etc).
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.