Loudspeakers for small medium big huge rooms.
Do you want to calculate the necessary speaker size yourself? No problem.
The starting point is the desired sound pressure level at the listening position. The listening position is assumed to be far away from the loudspeaker:
If the floor standing speaker is very close to the wall, a wall loudspeaker may be recommended instead
To complement a floor standing speaker use a subwoofers.
In large rooms several subwoofers are recommended.