To choose the length for your aerial hammock you can measure your hammocks out based on height of the rigging point. Usually it recommend for your hammocks to use a standard 7 yards of fabric, it's about 6.5 meters. Next you have to adjust the rig points higher or lower by using daisy chains or other means of adjustment.
To rig your hammock at home for trainings you may decide that you need only 4 yards of fabric, approximately 4 meters. The calculation really depends on your ceiling height and should be based on rigging points availability. But, even though it might seems a lot, it better to order 7 yards of fabric anyway, because you will have enough fabric to tie the knots required to safely rig your hammock.
You can rig a single or double point hammock. It's absolutely personal preference. The choice can be based on your performance needs or yoga practice, and how you utilize the hammock. Definitely, if your routine require spins, you have to choose a single rigging point for your hammock. For beginers or if you need hammock for yoga practice a two point rigging will give you a rigid structure and more stable position comparing to single point.