Whenever you organize events like this, there is never enough time or space for all the wonderful sessions and people. Lightning talks are a great way for lots of participants to get exposed to lots of content, people, and projects.
The format I've seen used to great effect is:
- five minute talks (maximum)
- demo/deck required (at least one slide if nothing else)
- half pre-event sign-ups (with voting if possible), and half day-of sign-ups for presentations
Whatever format the community decides, lightning talks are usually well attended, often light-hearted and/or novel, and a great way to start and/or end the day.