to me, the "smallest viable mat size" could be one machine, if that machine could do enough to justify my investment and returns.
imagine if this one machine (i.e. washer) could just do 3 turns per hour or 72 turns per day, that would be much much better than having 72 machines each doing one turn per day?
besides, one machine will cost much much less than 72 machines, and getting you the same results
what should matter is the bottomline?
should your decision be can you make the money you want in that laundromat or machine?
also you can learn a lot on a website like this, even without owning a laundrymat

i'm sure there are people on here who can teach you how to take a part a laundry machine as well as put that machine together, or fix just about everything in a laundrymat. just ask your question....
i'm already becoming an expert in fixing coin jams, adjusting dryer temperature, etc

....
over the weekend i dismantled a toploader, and i was amazed it's nothing more than a tin-can for the drum