To me this is hard to pin point a number. I feel that if Masonry, real Masonry is being taught the number can be as large as not to hinder this. I would like to brainstorm on ideas to ensure active membership. Things like you MUST attend 3 masonic functions during a year to retain your membership. It does the craft no good to have people carrying cards and not be active or have a clue. (I know there are good masons that are not active, this is more directed to the Brother that was raised and has never stepped foot in a lodge again). Now keep in mind I know that is not going to ever work because you have older members that can not travel but that is an idea.
To answer your question, if Masonry (not just getting together) is still effective then the Lodge can grow. At the point it become ineffective and politics plays a larger part than Masonry then it is too big. If a Lodge can not open or there is no additions to the line up in the last decade then we really need to analyze that lodge a little better. See if Masonry is still shining there.
I really don't know if this is ideal but its brainstorming.