This OpenSeadragon ( http://openseadragon.github.io/ ) plugin provides a scale bar which adjusts depending on the zoom level.

A demo is available here: http://nistdeepzoom.github.io/OpenSeadragonScalebar/

It can be used like this:

var viewer = new OpenSeadragon.Viewer(...);
viewer.scalebar({
  minWidth: ...,
  pixelsPerMeter: ...,
  color: ...,
  ...
});

To change any property, just call viewer.scalebar with the updated property. For example, to change the pixelsPerMeter:

viewer.scalebar({
  pixelsPerMeter: ...
});

The list of all the options can be found and tested on http://nistdeepzoom.github.io/OpenSeadragonScalebar/ If type, pixelsPerMeter or location are not set (or set to 0), the bar is hidden.

Disclaimer:

This software was developed at the National Institute of Standards and Technology by employees of the Federal Government in the course of their official duties. Pursuant to title 17 Section 105 of the United States Code this software is not subject to copyright protection and is in the public domain. This software is an experimental system. NIST assumes no responsibility whatsoever for its use by other parties, and makes no guarantees, expressed or implied, about its quality, reliability, or any other characteristic. We would appreciate acknowledgement if the software is used.