Gneik has a static back length (non-adjustable). An RS (Regular Short) "ideally" fits a back length of 43 cm and an RL (Regular Long) fits "ideally" a back length of 48 cm. Of course, there is a tolerance both upwards and downwards of a few centimeters.
If your back length is between 43 and 48, you can bear in mind that the width / length of the shoulder strap and the length of the hip belt differs between RS and RL (wider and longer on RL). If you are heavily built, an RL probably fits better and if you are thin, an RS probably fits better.