Answer: Largest Protest at a Convention

I expect there is no firm answer. It's not unusual for the media to report the number of protesters as far fewer than participants estimate, sometimes even half. Just because of the ever-increasing population, I would expect that the more recent conventions have seen the largest protests.

In 2000, the DNC in LA drew "tens of thousands of activists from across the country came to Los Angeles to protest against the Democratic National Convention," according to organizers at the D2K Network. http://www.d2kla.org/

The number of protesters expected in 2000 by the LAPD was "anywhere from 30,000 up," according to an LAPD spokesperson.
Schools, businesses prepare for Democratic Convention protests http://www.cnn.com/2000/ALLPOLITICS/stories/08/11/la.protest.prep/

By comparison, at the DNC in Chicago in 1968, the "number of persons at any of the protest sites never exceeded six or seven thousand, including a thousand undercover agents and a number of spectators," according to the Chicago Public Library site. Also, 641 persons were arrested and numerous protesters, as well as 198 police officers, reported injuries."
1968, August: Disturbances at the Democratic National Convention
http://www.chipublib.org/004chicago/disasters/1968dem_convention.html

Thanks for asking Radical Reference.