Alexandru Vornicescu

If you want to resize featured images (post thumbnails) on the fly for your posts automatically then this plugin can help you.
To take effect just open a post or page and all featured images in that post will be resized to a normal size.
This plugin can be very useful after a theme install.
It can be an alternative for Regenerate Thumbnails plugin.
I think this plugin is better because you will not need to wait before all images are resized for all different sizes, just for images that are need to resize.
The plugin will keep the original copy of all image files.

Resize Post Thumbnails

This plugin is very useful and time-saving! Works fast and doesn't need any extra steps.

Best alternative to Regenerate Thumbnails

This plugin will resize post thumbnails on the fly.

Resize Post Thumbnails vs Image Optimizer, Resizer and CDN – Sirv