I'm putting a 36" linear in a wall that is 73", window to window. Most importantly because the existing gas fireplace is 36" and I don't want to pay to have all of that rebuilt. The current surround, builder special of wood sides and mantel and a bit of marble around the fireplace opening, is 60". We're flexible on the new surround - again wood with marble but at least it will be something that looks like it was designed in this century - but am sticking with the current dimensions or a smidge wider so no major drywall repair or painting has to happen to the large 2-story wall.

This project will start...soon. Down payment made in February, along with full payment for fireplace which I don't think has yet arrived so I guess the sliver lining is that I haven't had a gaping hole in my wall during all of this stay at home and contractor not working. A 36" linear is also exponentially less expensive than anything larger, but if you're rebuilding the whole box I guess you can choose what you want.

This is the 36" model we're getting. Someday.

That ^^^ stone wall is wider than 66" and wider than my planned 60" space, so the fireplace will not look as "small" as it does here. Don't forget that your linear fireplace will be higher off the floor; I think mine requires installation at least 8" above the floor/hearth.

I don't understand the rest of your question; not sure how stairs and fireplace location work. But stone on stairs I assume requires some extra support?