How to change cms article preview size

For images:
Style Manager for your style, go to Stylevars, attachment_image_thumbnail_max
For videos:
Style Manager for your style, go to Stylevars, vbcms_article_preview_object_size

For changes to take effect you will have to rebuild all your image thumbnails and then clear the system cache.
If you get image errors like I did (Error:Can not determine image size) then got to your image settings and make sure “GD” is selected and try again to rebuild the thumbnails.