Welcome!
You'll need to figure out where the viewing area is. My sense - with a room that large - is that you will have adequate space behind the seating area to warrant 7.1.
One of the nice things about the QS speakers is that they are not fussy relative to placement. Generally, the surround speakers should be barely in front of or barely behind and to the sides of the main listening position. The Rear speakers typically go on the wall opposite the screen.
As to height, about 2-3 feet above the listeners' ears works well for both the Surround and Rear speakers. I put mine so that the bottom of the speaker is about 6'1" off the floor (that way, I don't ever bump my head; anybody taller than me can just be more careful).
Dolby has some excellent resources that show - with charts and everything - desirable angles and distances and stuff.
Wire or otherwise allow for multiple possible subwoofer placements. It's not practical for amateurs to model or project room nodes in open spaces ahead of time, so give yourself some flexibility, which can make a huge difference.
Have fun. Enjoy the journey.
