Determine the width (this is at the widest point) of the area. If it is less than 2m wide, you will need to round it up to a 2m width. If it is between 2m and 4m wide, then round it up to a 4m width. If your lawn is wider than 4m you will need to join 2 pieces of grass together.
If it is more than 4m but less than 6m, you will require a 2m width and a 4m width to be joined together.
If it is wider than 6m but less than 8m, you will require two 4m width pieces to be joined together, please bear in mind the grass is supplied in 2 and 4 metre widths.