Joe and Ken, thank you very much.
The sail is pretty much how you describe it, Ken: no laminate, small plastic window, no marking or number, and it's a grommet at the bottom.
Joe, I was dividing by 2, so your 1.5 is probably what I had wrong.
With a visual comparison (from memory) with the 4.5 sail, 5.5 or 6M would make sense.
I'll go with that.
Thanks for all the help, guys!