Personally 10 minets is rather good length, So you could shorten it down to 10 and then upload a longer version as well or load it onto disks for you presentation so if people want to watch the rest they can?
TWENTY MINUTES?! That means it's going to be even better! With documentaries like this I always think noooo it's not long enough! They could fit in so much more etc etc so 20mins is very very good Just make sure you shout about it on the internet loud enough that I hear as my memory is terrible atm and I'm really looking forward to it!