Hmm. This snippet is working for me in Reading view using the Minimal theme (Readable Line Length → ON).
If you can’t get that snippet working (and with it off), Minimal has Helper classes - Minimal Documentation 36 that you could use here. In your YAML, include the max cssclass:
Try:
---
cssclasses:
- dashboard
- max
---
This will change the width of Source, Live Preview, and Reading modes, but as it’s a dashboard note, it could be workable.