In highschool (199
I was part of the tech crew in the Drama Department. We had two hallways that we had to keep warm for the actors, both were 8' wide and tall. The little stage had a 25' hallway, and the large stage had a 41' hallway. It took one heater of similar style using propane to heat up the 25' hallway, but there was always a cold spot depending on where we put the heater. If in the middle, both outer ends would get cold.
The 41' hallway required that we used two and we normally spaced them about a third of the way down the hallway to heat it. We would normally get to about 72-ish degrees and maintain, with minimal b..ching.
Now I am sure that the technology of these heaters has come a long way in the 11-12 years since, but I dont know. Thats why I said to try one out first, then consider a second one. You don't load your plate sky high on your first time through, you can always go back for seconds. ^.^