Here's a link to the YouTube playlist of the conferences from the 2016 Championship. You can get a feel for the sizes of rooms used and how full they were (it obviously varies by presentation topic).
https://www.youtube.com/playlist?lis...o0k-k2pqBErS0i
And one of the presentations I know that list is missing:
https://www.youtube.com/watch?v=sJOfH-lomEQ
Topics span a great range. Most, but not all, are FIRST-specific (or sometimes FRC-specific or FTC-specific). Some others are tailored at interests that FIRST students have, but aren't explicitly FIRST-related (scholarships, NASA projects, tech industry, etc). The most popular presentations tend to be from individuals with high amounts of success within the FIRST community giving insights into particular aspects of FRC. The topic lists from 2015 and 2016 can be found below:
http://www.firstinspires.org/resourc...nd-conferences