Which decomposition option of the Chinese character 天 tiān ‘sky’ is correct?
大 is just a person standing, see the old forms of 大 http://www.zdic.net/z/17/zy/5927.htm

Old forms of 人 show a person with his back bent, maybe working in the fields or praying or bowing.

儿 shows 2 legs, draw them together and you have 人。

If you put a line on 2 legs, 儿 you get 丌， which is a pedestal or some kind of small table, not 'heaven'.

More interesting is what the ancients thought of when they wrote 天。

Perhaps they thought 'the one, 一 that is above and bigger than mortals 大' which neatly uses the senses of big and person in 1 character.