One of my favorite commenters, Madison Man, just happens to have an office window overlooking the scene:
The time is 10:30 a.m. Notices for the event said that the "doors" would open at 9:00 and the event would begin at 10.
Not quite a hotbed of Michelle-love, is it?
AND: MM tells me that is the same sized crowd that was there at 10 and that the event actually started right after the shot was taken. So this is, presumably, as large as it got, i.e., much smaller than I would have thought.