In reply to apneaquatic: About ten years ago the fashion was for one long prussic and one short. Now everyone I know who knows what they are doing (i.e. MIAs, Plas y Benin Instructors etc.) just use 2 or 3 short ones all the same length.
You want 5mm cord tied with a double fishermans knot with 20-30mm tails forming loops around 45-55cm diameter. Around 1.5 - 1.7metres for each loop should do.
The exact length is down to personnal preference but I tie mine long enough that I can just manage to loop it over my shoulder if needed. Others prefer them slightly shorter.
HTH
