I have change the thumbnail size of the featured image in Theme Options (to 640px for example). However it still shows the old small thumbnail size (100px square). How do I get the new size to work?
To solve this problem, you need to generate new sizes for all previously uploaded images. This process is often referred to as regenerating thumbnails.
You can install Regenerate Thumbnails plugin to quickly regenerate all your Featured Image sizes.
1- Go to Plugins – Add New
2- Search Regenerate Thumbnails in keywords
3- Click Install Now
4- Once installed, click Activate to activate the plugin
5- Go to Tools – Regenerate Thumbnails
6- Click the appropriate button and wait (do not close window) until it completed