Massive Stairway beneath Rigus, how big?

[Voadam]

In my campaign the high level party has just entered the 1 mile stairway beneath the Planescape gate city of Rigus leading down to the Lion's Gate to the plane of Acheron. They are engaging a bunch of dark dwarves about two-thirds of the way down the stairs and I'm trying to come up with actual dimensions for the stairway now that there is combat.

Rigus is a city dedicated to war and I pictured the tunnels as the Legion's Stair, big enough for an army of soldiers to march down to the gate.

Any suggestions on dimensions for the stairway?

Any actual references to such in planescape material?