I saw a place to set the excerpt length once, but can't find it again. I've been to every corner of this plugin, and don't see where to set it. It's not in the main plugin Settings. When you are editing a newsletter, there is the Settings tab on the right, but it only shows fonts settings. The Blocks tab only allows re-ordering of the blocks.
Okay, found it. You have to hover over the right upper corner of the newsletter body section (below the header) and click on the Ruler/Pencil icon (says "Edit" when you hover over it). Then the settings appear on the right.
This is hard to find, so the documentation needs to be changed to make this easier to figure out, I searched "excerpts" and found nothing useful.