Reading speed and font size
A spoken English pace of about 120–150 words a minute is comfortable for most talks. The speed slider does not count words; it moves the script in pixels. Start around 1.2, record a sample sentence, and adjust until you are not leaving long gaps or rushing the ends of lines.
Type size should be large enough that you can read a line in one glance at your working distance. On a laptop at arm’s length that is often 40–60 pixels. On a tablet a metre in front of a camera it may need to be larger. If you wear glasses, test with them on. Comfort beats packing more words on the screen.