Point sizes are relative to the font that's used. For example: 14 point Rockwell isn't as tall as 14 point Trajan. The size of the font on screen is a subjective matter. The only rule of thumb that comes to mind is stay clear of small fonts with thin serifs - the thin lines can get chewed up by the time it hits air. I've never had an issue with the text being to small. Oftentimes I want to make the text smaller because it looks better, but the client jumps in and says, "could you make the phone number bigger". "How's this", I reply after scaling it up 15%. Then they say, "make it even bigger". Oh boy, here we go again.
One other thing you could try it to burn the spot to DVD and play it back on a standard TV. All of the cable and satellite companies compress everything to MPEG for delivery. So if you burn your spot to DVD and the text looks a bit crunchy on playback you might want to choose a different font or bump up the size a tad bit.
Disclaimers (for political ads, etc.) have a minimum size. For example: 20 scan lines (as measured on a waveform). This height varies fro state to state.