Analyze > Tools > Scale Bar...

Most images have pixel size metadata, meaning no manual calibration for the scale bar is required. If you find that only a "pixel" value is displayed, this metadata is missing. In this case you may be using a saved image of the original (for example, a PNG file converted from the proprietary CZI or LIF file format). Revert back to the original.

A µm unit is encouraged, and should be displayed prominently under the scale bar. Ensure that you are saving images with scale bars separately from the raw image data. Any image prepared for publication, presentation or poster should have clearly-labelled scale bars.

Mac users may find that a "read only" error is preventing them from installing FIJI updates. This would also prevent new plugins from being installed. The error is caused by a Macintosh "quarantine" being placed on FIJI, and quite common. When attempting to install a plugin via the "Manage Updated Sites" function (Help > Update...) you may encounter this error (or something similar):

"[WARNING] Your ImageJ installation cannot be updated because it is read-only"

A solution to this has been posted on this page (Curtis Rueden, April 2020, thanks to Chris McCrory for testing this out). In short, launch your Terminal app, then paste the following code (assuming FIJI is in your "Applications" folder): 

cd /Applications/Fiji.app; sudo chflags -R nouchg .; sudo xattr -rd com.apple.quarantine .

The quarantine is now bypassed, and you can install updates as normal.