To increase the size of the thumbnails on a specific category:

In this example, we'll use a News content type. 
  1. In onMessage, edit the page that contains the News content type we want to edit

  2. Click Manage
    User-added image

  3. Click Settings inside the News category
    User-added image

  4. Inside the modal, scroll to the bottom and change the size of Thumb
    User-added image

  5. Note: Image include and size settings only apply to new photos being uploaded. Existing photos will not be changed, so you'll need to re-upload those photos to take the new setting.