NOTE: You do not mention which version of Twine 2 you're using and that's kind of important, since you're asking about Twine 2 features.
If you mean the story map, then the short answer is you cannot. Passages may be arbitrarily large and there's literally no way to show the complete text of any and all passages.
That said. If you're using, at least, v2.2 of Twine 2, then you can click on a passage's ••• context menu item and change the display size of the passage box to large. Unfortunately, there's still some internal limit, so that won't get you much extra text shown even when supported by the box size.