If you can t get access to the room in advance, find out how big it is and measure off that distance. In general, never use a font of less than 24. Sans serif fonts are simple and easy to read, but decorative styles of font can be tiring. Also remember that the lighting in the room can have a big effect on visibility. Darken the lights near the screen if you can.
Nobody can read and listen at the same time.
Instead of using long phrases or full sentences, use only a few key words. Also, limit the total number of slides. For a short speech in the 5-minute range, four or five slides may be a reasonable upper limit. This way, you avoid information overload on your audience. Don t let your visual aids take over.