1) What length should I choose?

Measure the circumference of your neck and check out the length in cm/inches. This will best determine which length between choker (12inch) to long (18inch). Every neck is different so its best to measure.

2) What lengths should I choose if i want to stack multiple necklaces?

You can choose the same length as for each necklace theres an adjustable chain that extends to the next size. If you have a three or more stack its good to have different lengths.