Poi tethers and more specifically poi tether length is something every poi spinner must figure out for their unique needs and style. For jugglers and contact spinners, poi leash length may be quite long whereas for tech spinners or people playing inversions, the poi leash system may be quite short. I'm going to try and break down the best methods for figuring out how long should poi tethers be and in the process hopefully give people a starting point on figuring out the answer to that age-old question: how long should my poi be?
In general you want to start off with a length of roughly that of your arm--essentially from your first thumb joint up to the crease of your shoulder. This is long enough that tosses and contact work should still be very doable and short enough that things like buzzsaws are still doable.
If you like poi juggling or contact poi, you should also give some thought to longer tethers as they give you a bigger target for catching your tosses as well as more momentum for your contact tricks.
For people who prefer inside tricks like lovelacing, inversions, or toroids, a length from thumb joint to mid-biceps or even elbow is a safe bet.
Bottom line: start out with your poi at arm's length and see what tricks you want to play with. Then switch up your poi length to suit your style as it evolves!