Upvote:2
Translated suttas talk of a "roar" in an assembly -- maybe that's metaphorical but maybe not entirely.
I think of a crowd of 500 as being large. Maybe with that many you choose a suitable venue (unlike other suttas which are dialogs with a just a few people). The acoustics of the place could be important. The Ancient Greek Theatre of Epidaurus has famously good acoustics and seats 14,000 -- so 500 is plausible.
500 seated one per square metre is a semi-circle with a radius of 20 metres. I think that being heard at that distance is just raising your voice, talking loudly, not even shouting.
This paper claims that the number of people who could hear speeches at the forum at Rome would be between 400 and 1400 -- standing -- depending on how noisy the crowd was:
I think that (the Roman forum) is bad acoustics, with stone walls echoing. So an open-air speech might be easier than that.