As soon as you have a stream going out from OBS, the video wall doesn't matter. Completely separate problems.
Set up the video wall on that machine, how you want it, then see what overall frame size it wants and how it wants to receive the stream. Set OBS to that frame size, both canvas and output, and a custom stream like the wall wants. Then build your scenes in OBS with that frame size.
If you have something different on each display of that wall, then you probably don't need OBS at all. Just maximize each thing on its own monitor and be done. OBS *might* be a translator for one or more of them, if they don't have their own display apps, using the right-click -> Projector function on either the Program or an individual Source or Scene, each sent to its own fullscreen monitor, but that'd be about it. All on the same machine, no stream.
If you've painted yourself into a corner with a wall that *must* receive a stream and can't do anything else, and doesn't differentiate between individual monitors, then you're back to the first idea to build a scene in OBS with the full frame size, and know for yourself where the boundaries are (pixels, not a physical ruler), so you can size and position things accordingly.
