If you understand the principal (not soundstage shape?) then you can answer your question. The dimensions can be manipulated using the ratio as Cardas has done. Sanders Sound has different approach; I hear so do others with using small asymmetry from side walls.
Crazy Cardas?
There is a web page from Cardas that discusses speaker placement that seems to be a bit different than most. The most shocking is for the placement of speakers from the wall behind them. In my 15 foot wide (rectangular)room, the Cardas formula calls for my speakers to be out about 80 inches!!!! It also calls for an equilateral triangle for the listener and speakers. This puts me 55 inches away. Am willing to try anything for a little while but have never spent any serious time on nearfield listening. Have I made some significant error? Am currently listening on Salk Songtowers(the rule is different for planar speakers). Google Cardas and Golden Ratio to find web page.