What seam allowance are you sewing? A 1cm seam takes a lot more of 6.5cm block than a 1cm seam from a 6inch block.

if you have a 1cm seam: 4 blocks x 6.5 will give you total of 20cm (5.5 + 4.5 + 4.5 + 5.5 cm)
if you have a 1cm seam: 7 blocks x 6.5 will give you total of 33.5cm (5.5 + 4.5 + 4.5 + 4.5 +4.5 + 4.5 + 5.5 cm)

Can you add another row across, and 2 down (to make 5 x 9 blocks)? using 1cm seams this will give you finished 24.5cm x 42.5cm