What size expansion gap should I leave between my wall a hardwood floating floor?
Your vapor barrier needs to extend beyond the edges of the floor to prevent moisture from getting to the floor.  Install in accordance to manufacturer's recommendations.

The expansion gap needed also depends on the manufacturer's recommendations - different materials expand at different degrees.  The point of the gap is to give it room so it doesn't end up buckling.  If you can't find a solid recommendation then use 1/2" for the maximum protection - and if the trim you install doesn't cover the gap, use quarter round.