36 bold font for titles, 24 bold font for text in the body of the slide!
OK, I confess to using 32 bold for titles or 18 bold for text on the slide, but I try to avoid it. Also use a simple clear font, like Arial, Helvetica, or Times. Nothing fancy. You should assume that anything less than 18 bold is essentially invisible to the audience.
There are two reasons to follow these rules: 1) the obvious one is to help the audience see, and 2) the less obvious one is the discipline it poses on the presentation creator. Now I can hear you now, "I can't possibly get the content I need on a slide using these rules." However you are trying to get too much on the slide. Figure out the point the slide is trying to make and only include words that help you make that point. Also you don't need complete sentences. A slide presentation should be the synergy of audio and visual. Words on a slide are simply cues that you amplify with the words you speak. Take some time to edit down your text.
Occasionally you may need to illustrate something with smaller fonts (e.g. copying something from another source). In that case point to what you've copied and tell the audience that they are not expected to read the detailed message. If they are, then you need to blow up the image. You don't want the audience struggling to read small fonts.