There are some charts available online that offer a static indication of sword length versus body length. While they sometimes may work for modern ZNKR Iaido schools such as Muso Shinden Ryu or Muso Jikiden Eishin Ryu, the are too generic and don't count numerous relevant factors.
In ancient Japanese martial arts the correct length of ones sword is determined by:
- the persons length
- the length of the arms & legs
- the ratio between lower and upper body
- the flexibilty of the wrist
- the width or the hand
- the Japanese sword school (ryu-ha) that is practised
The best way is to consult your sensei and get advise of which type of sword and sword length suits you best.