You can reset the Lightroom Preferences back to default and see if that helps.
For resetting Preferences, please refer: Preference file and other file locations | Lightroom CC and 6
Let us know if that helps.
I have the same issue, but for me I think it has something to do with Display scaling. I have a 3200x1600 monitor, and the first week using it Lightroom's File menu fonts were quite large. Suddenly last week those got better but then the Library Develop Map . .etc got huge. Perhaps it is the way Lightroom is interpreting the scaling from the OS (mine is Windows 10).
If I go to Edit > Preferences > Interface > Font - medium or large, it fixes it. But it would be great if the automatic setting could handle it as usual.
Thanks for the update, As of now the UI Scaling is not yet introduced in Lightroom preferences. You may do the feature request for the same, refer: Feature Request/Bug Report Form