Hi Ariel,
Thanks for your question.
My first comment would be that you should not try to manage a group of 20 participants if you are just one facilitator – it doesn’t make sense. Try to find someone else, preferably LSP-qualified to support you.
With regard to managing the size of the group, I think what most of us do is to distribute participants into tables of 4-6 (4 being better than 6) and provide opportunities for tables to present their stories to each other and for participants to switch tables, after a sharing of stories.
As far as I can see, the idea of gradually halving the number of groups / doubling the number of participants per group would just slow down the process. Try to use the World Café principle of keeping at least one host at each table throughout the process, while allowing others to move around at given points in the workshop.
Yes, it is possible to run a workshop with 20 people single-handed, but why would you put yourself through that sort of stress, when you have so many colleagues here who could support you?
All the best,
Eli