rtmp-module, push livestream chunk_size and record on

I setup an nginx server compiled with the rtmp-module and I am currently just using push to push my gaming livestream to both hitbox and twitch. I noticed some people use
[code]chunk_size 4096[/code]
and some people use
[code]chunk_size 8192[/code]
I tried googling the nginx chunk_size but didn't get many results. Can someone please tell me what this setting does, I would like to optimize the use of nginx with the rtmp-module the best I can.

Then my second question is how to enable nginx to store a local copy of what it's pushing. I tried to enable
[code]record on[/code]
but then nginx fails to start