By default, all the image thumbnail options are disabled, but you can choose what size you want to use and is based on whatever Blog Style and Layout you choose.
NOTE I should mention that if you choose not to create thumbnails, any photo you upload will be used in the size and orientation that you originally uploaded.
- Go to Appearance >> Customize >> Thumbnail Options
- Make your choice from the listings of thumbnail options. I recommend only selecting one based on the blog layout you plan to use, otherwise you will end up with a bunch of thumbnails that you don’t need.
- Click “Save & Publish“
Featured Image Size Reference
This will help give you a sizing reference if you decide to enable any of the options below. If you leave them unchecked, any featured image you upload for a blog post will be the size and orientation you upload.
- Default creates featured images at 960×535 pixels
- Card creates featured images at 700×500 pixels
- List creates featured images at 700×500 pixels
- Masonry creates featured images at 700px wide and any height relative to the image
- Related Post thumbnails are not a blog, featured image but this does create the related posts images on the full post view at 320×200 pixels
Depending on the blog layout you choose, I recommend using the best photos as possible when uploading and to make sure they are at least the sizes mentioned above. Images will be cropped, so that is something to also remember when deciding on what kind of photos you plan to upload.
Re-Generate Thumbnails
There may be times you need to re-create your featured images with the cropped settings, either from the theme or from the WordPress Media Settings. The most popular plugin to use is called Regenerate Thumbnails.
This let’s you re-create the corrected sizes…providing your original image uploads are large enough they can be cropped correctly.