What shud be the maximum height of rear speakers .
Will the sound quality get affected if I place rear speakers at 8 feet(ceiling).?
Is there anyway I can place the rear speakers down if I don't have a rear wall?
General rule is it should be at your ear level, when you stand. i.e around 5-6 feet. I have placed my rear speakers at the same level. But I'm yet to check the sound effects still. If there is no wall you can use a stand
However in one of my earlier dealing with a Bose Enginner they recommended that they avoid keeping the speakers midway between ceiling and wall and place towards the floor or ceiling like in your case but I have not compared the difference between the two.
these surround speakers are directional. if they are not bipolar / di pole surrounds you might have a problem placing them 8ft higher or lower than your ear level. as mentioned by @thirurajesh best bet is to place them at ear level when you stand and point them towards you if they are bookshelf surrounds so you get the best surround effect. or use a stand