There's a feature in Keynote that lets me indicate that any text inserted into a box will be "shrunk to fit" the box, so that any amount of text will fit into the box and always be displayed (albeit very small if the text is very long.)
It reminds me a little of the KM option called "display large" in the "Display Text" action, however Keynote (KN?) supports line wrapping while KM does not. Line wrapping is important to me. And the KM way of doing things doesn't give me any control over how long the large message lasts, or what it's transparency should be.
If I want to do text wrapping combined with "shrink to fit" in KM, it seems I'll have to do it manually using the action "Composite Text onto Image" but that's not very effective because the action doesn't tell me how far across the image the text was successfully written. May I request a new KM token called %CompositeTextWidth% which returns the text's output width of its most recent action? In a way, this is in keeping with the approach of some other tokens like %FoundImage% which also returns useful data from an action.
I found a way to generate styled text of a given font size using the textutil command in macOS. That could help me a lot, but it still won't tell me how wide the text became when I wrote it into an image. So it's only half the solution to my problem. But here it is for those who are interested:
textutil -stdin -stdout -convert rtf -font Arial -fontsize 32