The most important function of a prompting system, whether its an oncamera or speech prompter is its readability. And yes, size matters. Compare the easy to ready 12-point font of a childs book versus the 6-point font of legal fine print. So setting the size of the text being scrolled is a good place to start within limits. If the font is too large, it will result in too few words per line the speaker may not be squinting, but the chopped up text will result in broken words and sentences, and more importantly, rough delivery. Conversely, too many words per line will result in noticeable side-toside eye movement the right number of characters per line for prompting ranges from 16-24. In general, you may find it desirable to use plain fonts, as opposed to the more ornate fonts, and select sizes from 48 to 72 point in size.