It should be 0 - no world government. Please
This of course, but that ruins the thread, does it not?
10,000 is probably close to too much, if you literally think about getting all of these people in one place. Although, I suppose that might not be necessary in modern times? Also, if all of these members will need to be paid a full-time salary, that would cost several hundred million in and of itself (well over a billion if we pay them a similar amount as US representatives--money that, while admittedly dwarfed by government expenses generally, would make a tangible difference put to other goals).
When it comes to legislature size, on one hand I like a larger number since it means more chance for a variety of views to be represented (I mean, in the US it's "nah let's still do FPTP" and you have ~200 people for ideology red and ~200 for ideology blue, yes with a few variants as "token complainers" while most people get locked out of the system--but I'm not derailing the thread with a rant today
) but on the other hand I'm concerned that a larger chamber means it's harder to hold individual people accountable.
Regardless, the thing Crabcake mentioned seems like the best idea.