To set the global contest settings which apply to ALL contests go to Contest > Settings
Please note that the available sizes to choose from are populated from your WordPress Media Settings:
WordPress Dashboard > Settings > Media
This ensures that your contest will better match the rest of your website.
You can override the global settings per contest by specifying the image height and width in your Shortcode.
Example: showcontestants id=42 height=200 width=200
Facebook Settings, Twitter Settings: