I find that soundstage size is a direct function of the placement, but imo the equipment/electronics dominate the matter of depth of soundstage.
One of the variables that influence depth of soundstage is the sense of width. I use the PureAudioProject Trio15 Horn 1 Speakers (see my reviews at Dagogo.com on the variants of this speaker) both Portrait (vertical) and Landscape (horizontal). The soundstage changes dramatically with these two options. But, still, the biggest impact on depth is the electronics and cables.
One of the variables that influence depth of soundstage is the sense of width. I use the PureAudioProject Trio15 Horn 1 Speakers (see my reviews at Dagogo.com on the variants of this speaker) both Portrait (vertical) and Landscape (horizontal). The soundstage changes dramatically with these two options. But, still, the biggest impact on depth is the electronics and cables.