The font that you are going to use should be legible. I would recommend Arial
or anything that is not very fanciful. As for font size, check this out: http://www.rdpslides.com/pptfaq/FAQ00125.htm
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Shawn Toh (tohlz)
Microsoft Most Valuable Professional (MVP PowerPoint)
Site Updated: Sep 01, 2006
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For me Arial and at least 30 point. Sure there'll be other views!
Colour though is also vitally important. A good contrast with the background
is needed and some colours dont project well on some projectors (yellows
often but not always) Try it on the projector you'll use
I just did this for a group of 600+ in a Curling Rink which is quite large
and had no problem viewing the presentation anywhere in the room using 50 as
my largest and 30 as my smallest font size. A lot depends on the lighting in
your auditorium