Screenshots from WordPress’s Regenerate Thumbnails plugin:
This plugin works in tandem with your theme’s “functions.php” file…
See where the size of the thumbnail has been set to 300 wide and 111 high… the “true” is for the “crop” tool. The “add_image_size” is for the size of the image on the full post page. You can probably use it elsewhere too. I’m not sure how many sizes can be declared for use.