Someone who's going to deliver a 25-min lecture should find no difficulty writing more than 300 words to give a taster of the lecture. The speaker will typically monologue for 17 minutes, which would be about 2000 words (assume typical English speeches in Wikimania). 

... but the audience will pay attention for 400 words and probably start checking their emails after 1,000!

Please treat this email as a cry for interactive sessions and not people reading from a Powerpoint!

Chris :-)