The Upstairs Theatre looks different for each play. The banks of seats are mobile and can be re-set. For a play like Straight White Men they formed a proscenium stage theater. For Lord of the Flies, there were seats on two sides of the space. For other shows, there would be banks of seats on 3 sides of the space creating something like an extreme thrust stage. However, there are always 235 seats.
Steppenwolf has been creating a new performing space -- the Round Theater -- just south of the main building on Halsted St. where the street level parking lot and Garage Theater used to be. When it's finished (it may be complete now since I haven't been there since Feb., 2020), they will re-purpose the Upstairs Theater into a rehearsal stage.
I suspect this expansion has contributed to the demise of the Royal George Theatre, which is located on the east side of Halsted (right across the street).