I think a design meeting like this is akin to a production meeting, but while ideas are still being thrown about - not always the best place for a stage manager to be.
I feel like directors and designers should go through a "out of the box" period where they are allowed to come up with wild, budget breaking, crazy ideas to riff on - and not be reigned in. Then as they get closer to design deadlines, the reality of the production limitations should kick in. Given that most of the time a Stage Manager isn't hired until pre-production, often the design process is all but over.
If you are lucky to be a part of the discussion for the design process, it might be good to have all the data you need for the show the performance space - number of dressing room, line sets, dimensions of the space, inventory of in house stock - and maybe help them get on to solid footing as far as a realistic design is needed. Having a character/scene break down, location of each scene might be helpful as well. Also knowing your crew size maybe helpful.