A very helpful website we recommend using for CSS is http://www.w3schools.com/

CSS Code for max height:
max-height: 100px;

CSS Code for max width:
max-width: 100px;
  1. Open a style in onMessage
  2. Select Content Types in the drop down
  3. Select the photo icon
  4. Select the Advanced button in the lower left of the screen
  5. Drop the above CSS code into the space. 
  6. Adjust the px amount per width and height as needed
  7. Save