How can I have smaller dimensions for the default image?

If you use a large Open Graph Image Dimension, as suggested in the plugin help / documentation, Facebook will feature that image more prominently in shares. In cases where a post/page does not have an acceptable image (missing and/or too small), the default image is used. If that default image is a logo, you may want Facebook to include it more discretely in your shares. To do this, you should use the “Default Image URL” option instead of the “Default Image ID”. Enter a URL to a smaller version of your logo (300x300px for example).