Screenshots from WordPressâ€™s Regenerate Thumbnails plugin:
This plugin works in tandem with your themeâ€™s â€œfunctions.phpâ€ fileâ€¦
See where the size of the thumbnail has been set to 300 wide and 111 highâ€¦ the â€œtrueâ€ is for the â€œcropâ€ tool. The â€œadd_image_sizeâ€ is for the size of the image on the full post page. You can probably use it elsewhere too. Iâ€™m not sure how many sizes can be declared for use.