The Smooth Height setting is what controls the sizing of the Carousel items. To change this, navigate to:
  1. onMessage > Website > Site Name
  2. Select the Styles tab
  3. Search for the name of the style and click Edit for that Style
  4. Click the Gear Icon and click Edit Settings 
  5. Change Smooth Height to Off
Note: The carousel height will automatically adjust to a fixed height of the tallest item in the carousel