Where possible, try to find a dining bench equal to, or slightly smaller than, the distance between your table's legs. This will allow you to tuck it neatly underneath when not in use.
Most dining benches are between 18" and 20" high. This allows plenty of legroom for your seated guests.
Yes, in two ways. Firstly, if your bench is placed against a wall, the table can be arranged closer to the wall, which opens up more floor space. Secondly, chairs can occupy a large area, so where three chairs may be positioned, four adults or five children could be seated on a bench.