Words, and nothing to fiddle with
You can’t change typefaces, sizes or colours. The only formatting is emphasis, for a word you want to stress: ⌘B on the Mac, or Emphasis in the edit menu.
Spelling mistakes are underlined, and nothing changes until you pick a correction. Apple’s Writing Tools are switched off for the speech, since they can rewrite whole paragraphs.
Until you rehearse, the length is an estimate based on a typical speaking pace for the speech’s language. After that, it uses your own times.