The auto-size feature tries to maximize the font size while still fitting all of the words on a single slide. It does this by calculating and dynamically changing the font size based on the number of characters that will be printed on each slide. In general, the more characters on a given slide, the smaller the font size. It's not perfect, but it fits all of the words on a slide in most cases - unless perhaps there are simply too many lines/characters, etc. per slide. I ran a test with "Crown Him with Many Crowns" and noticed that when the auto-size lyrics font setting is enabled, the effective font size for both of the generated slides was size 32 (bold). If I disable the auto-size feature and use the same font size (Arial 32 bold), selected manually via the format menu, then I get the same result (i.e. the slides are generated with the same font size, etc., and the slides appear identical to when the auto-size feature is enabled). However, if I turned off the auto-size setting, and used a font size of Arial 36 bold (or larger) the words did not fit on the slide(s).