Reader David Thomasson writes in:
Here's one way to think about it: Those who estimated 60,000 to 70,000 would have us believe that if a capacity crowd from the Orange Bowl would fill the yellow areas shown here.
He supplies this schematic, which suggests far more than the 75,000 estimate floating out there:
And there's this Pajamas Media analysis, which yields an estimate of 850,000, the same number apparently that actually showed up for Obama's inauguration (after initial reports of 2 million). Having been in DC for both the 9/12 march and Obama's swearing-in, there's no question the latter event was much, much more populated. The article is very interesting for its history of estimates of attendance of Mall events.
I don't feel like I've got a real handle on numbers and estimates like this, but those lower six-figure figures (anywhere from 100,000 to about 250,000) seem to be pretty plausible.