This site estimates European Jewish population in 1935 to be 9.5 MM, about 1.7% of Total European population. It is unlikely that the Jewish percentage population in North America was much different than that, and worldwide would have been much smaller still.
In regards to a boycott organized by such a small percentage of the population, I see a key prerequisite to it becoming more than a trivial nuisance:
- The organizers of the boycott would have to have been seen as an elite to be emulated, much like modern Hollywood actors.
Given the pandemic anti-Semitism of the time, prevalent even in (more enlightened than Nazi Germany) countries such as the U.K., this criteria was certainly not met.
It seems to me that this was a gesture known, even by its participants at the time, to be ultimately futile, but which was driven by the need to make a stand somewhere.