The primary flute relationship
It is good practice to concentrate your learning efforts over a long period of time on one shakuhachi. The instrument should be of such a quality that the horizon of possibilities keeps moving back as you become more experienced with it. It can be a simple wooden or plastic flute initially.
As you become more experienced, your playing will be given more space and depth if you use a superior flute which fits you well physically and allows you to explore subtleties in timbre and dynamics. Length isn’t so important, though it’s probably accurate to say that becoming an adept player involves study with a teacher, and the standard 1.8 length is most often used in teaching sessions. It takes a long time to get to know a shakuhachi, and a good instrument will pull you upwards as a player, simply by revealing the endless depths of your relationship, over and over again.
When the going gets tough, for whatever reason, it's worthwhile to keep playing through your difficulties on the primary flute, rather than be distracted by other lengths at that moment. Tough times are the deep learning times.