Under the heading “Creature Size”, under the heading “Movement and Positioning”, in the chapter titled “Combat”:
Each creature takes up a different amount of space. The Size Categories table shows how much space a creature of a particular size controls in combat.
The table then gives square dimensions for each space. For example, for Medium creatures it gives a 5 by 5 foot space.
Also, directly below that, under “Space”:
A creature’s space is the area in feet that it effectively controls in combat...
None of this is part of the “Playing on a Grid” variant, which is in its own sidebar and mostly has to do with scale, movement, and range.
There is plenty of terminology in the books. Try reading them instead of pretending you’re some kind of spokesman for 5e. We’ve all heard the pre-release advertising.
No, I’m not. (See above.)
Closing your eyes (or turning your back) does not negate line of sight, so the only impact that would have is to add any extra effects that would result in. Personally, I would be disinclined to impose the blinded condition just because someone closed their eyes for a moment. I don’t use facing either.