We always recommend measuring your table before ordering but as a rough guide:
Most square 4 seater tables need a 1.5m piece
Most 6 seater rectangular tables need a 2m or 2.5m piece
Most 8 seater rectangular tables need a 3m or 3.5m piece
The above is meant as a guideline only, it is always best to measure your table before ordering and here is how to do it: