How big does a garden room need to be for a pool table?

For a 7ft English table played with a full-length cue, around 5m by 4m internally. The rule is the table plus a cue length plus a little clearance on every side. A 7m x 4m garden room is a popular size for exactly this reason, since it leaves plenty of space behind the table for a bar area or seating rather than filling the whole floor with the game.