Normally you choose the length of your sword based on your body length.
Ideally you can double check with your sensei. He knows best what katana fits you and the style you practice.
The table on the right gives you a good idea about the length you should choose for a katana or iaito
|Body length / cm||Body length / ft||Blade length|
|up to 170 cm||up to 5,58 ft||27 inch blade|
|up to 180 cm||up to 5,90 ft||28 inch blade|
|more than 180 cm||more than 5,90 ft||29 inch blade|