I do design studio sessions a fair bit — for what it's worth this would be my advice:
I want to, of course, prevent groupthink, so should I even be participating as people might tend to see me as the expert and skew their answers towards my expectations (even though I'll address them as critical observers)?
I think it's fine for designers to participate. In fact I personally try and get everybody from the product team involved, and if that isn't possible at least one from each "role" (dev, design, test, pm, etc.)
However I would not recommend participating if you're facilitating the session. Because:
- Facilitation takes effort and attention. You won't have enough spare if you're also sketching.
- The facilitator is a position of power in the session — it's hard to impossible to both facilitate and contribute in a reasonable way.
It's the "facilitator" bit that's problematical — not the "designer" bit.
Am I correct to assume that the output, being a raw sketch of an idea, is to be used as a start for the real iterative process, for which I use these ideas to steer the first mock-ups and flowcharts? Or should I take the output directly to the field and see what happens?
It really depends on the team, the stage of the design you're at, and the scope of the problem.
I've seen some good cross-disciplinary teams jump straight from the output of a design studio session to some quick paper prototyping sessions with users, and then straight to product. Because they're very, very good at working together, have a good grasp of the user problems, and because the scope of the original problem was pretty small.
In other cases their has been a lot of iteration on the solutions that came out of the design studio — indeed often further sessions once ideas get worked up more and tested with actual customers.
Also remember that the output of a design studio session isn't really just the final sketch(es). It's the deeper understanding of the problem and customers by all of the participants. It's the alignment among the whole group on the best options to explore first. So you need to be very careful if you take work away and work on it solo for extended periods that you don't lose those benefits.
Also, from what group size should I start splitting up? I think I'll a have about 9-10 participants from various backgrounds...
I tend towards smaller groups. With 9-10 participants I'd probably split it into multiple groups of 4-5 people. Because of the time that the pitch-critique element of a design studio workshop takes.
If you're time boxing your pitch-critique to 3 minutes per person then with 9-10 participants that's half an hour of pitch-critique in each round.
That kind of time slows everything down, and means that participants find it hand to remember the issues that folk earlier on in the round brought up. You lose the fast-paced nature that helps people keep throwing out ideas.
With 4-5 people you're looking at a much more reasonable 12-15m. You also have multiple groups which means another opportunity to generate different subsets of ideas.