There are two ways that this can be done.  

A. The first is documented here.

B. The second does not require modifying the CustomStyles.css and just requires doing the following:
  1. Go to Constituent360 -> Social Media ->Progress Meters.
  2. Copy the Progress Meter you want to resize.
  3. Edit the new Progress Meter and download the Initial Progress Meter Image and Final Progress Measurement Image.
  4. Edit the downloaded copies of those images with a graphics program like Paint or GIMP and resize them to specifications you are needing.
  5. Upload those two images.
  6. Edit the new Progress Meter and add the resized versions to their respective images.
  7. Save and replace the old Progress Meter with the new one where needed.