In case you were wondering, the Rivendell rain poncho seems to fit – more or less – even if you’re 6’5″ tall and rather corpulent.
It might be a tad too big for the Sloth, though.
I haven’t tried it while bicycling yet. I’ll report back when I do. The only complaint I have so far is that my shoulders are a little too wide for it, and so it kinda bunches up on top.