Featured Image

In order to have a thumbnail image shown on the posts page, it seems I need to set a featured image. However, this featured image then shows up very large at the top of my post in single post pages, how can I stop this from happening using code? I do not want to use a “hide featured image” plugin as I am currently doing as this affects the SEO.
Thanks!

https://wordpress.org/plugins/wp-colorbox/