If Stalin, in particular, was unaware of the extent to which FDR was handicapped, how did the US keep the Russians from realizing this during meetings between FDR and Stalin, like at Potsdam or Yalta?
Stalin and Churchill would certainly have known; the wheelchair presented a logistical challenge at several of the conferences (Yalta for instance).
There's mention made of some of that throughout the Liberation Trilogy by Atkinson, and I'm sure I've seen it in some of my reading on the war in Russia (again as it relates to the various conferences attended by the British, Russian and Americans).
I can't say at all how wide spread the knowledge was among the rest of the world though; did Hitler or Hirohito or for that matter Sikorski know? I don't know.