When designing your presentation, design it for the person sitting at the back of the room. This means that your font size should be large enough for the person sitting furthest away from you to read clearly. If people have to struggle to read what you’ve written, they’re going to stop trying and you’re going to lose their attention.
For example, look at the slide taken from Rachel Botsman’s TED talk called “The currency of the new economy is trust.” On the slide, Rachel displays a quote by Mark Pagel. Look at how large the font is to ensure that it is clearly readable from the back of the room: