Our experience suggests that eight members is the ideal group size, even though as few as three can also work well.
With eight members, each member's sense of their life's purpose will be reflected back by seven different people. There is a purpose in itself to let the group form without any other criteria than common time availability and language. Defining additional criteria for membership, e.g. people you know or people from your part of the world, will reduce the opportunity for "the hand of God" to offer multifaceted reflections back to you.
More than eight members may also work well, although it will be more time consuming and it may be more difficult to create a shared resonant field for all.