Font size depends on volume of content you need to fit into the slide and the final use of presentation whether it's going for TV display or general presentation or for printing. Better to keep the minimum font size around 10pt if too much content on the slide else keep around 14pt. read more
Font sizes for PowerPoint presentations. Font sizes for PowerPoint © Wendy Russell Don’t use anything smaller than an 18 point font – and preferably a 24 point as the minimum size. Not only will this larger sized font fill up your slide so there is not so much empty space, it will also limit your text. read more
I would prefer to see most text at a 28 or 32 point size, with titles being 36 to 44 point size. The only reason I would use a font less than 24 point is when adding explanatory text to a graph or diagram, where you could use a 20 point font size. read more