Currently, on de.wikipedia.beta.wmflabs.org the font size in NWE with a monospaced font is too small. On de.wikipedia.org the font size is larger and much more readable.
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Just to clarify: The font size dropped from 13.3px to 12.3px in 1.29.0-wmf.15. But since this version has been now live on Wikipedia for more than one week and nobody complained, this either affects only me (and users with a similar configuration), or nobody else cares about the change. In either case feel free to decline this task, if you think the font size in NWE is big enough (especially compared to the font size used elsewhere in the interface).
I'm currently seeing 13.33px in Chrome, but 12.3px in Firefox, so I'm guessing this is the double-monospace rule bug in some form.... In the old editor Chrome is 13.3px and Firefox is 12px - I also see no change if I go back a few release.
FF was 12px is 1.29.0-alpha, so I don't think there is a bug here. I wouldn't object to standardising font size across browsers, but that would be a different bug with a wider scope (both old and new editors).