Media Settings
- Go To : Settings > Media
- Specify dimensions.
Ok to keep default ( as per in screenshot below ) for most of our themes. Refer theme specific help for more details. - One more image size is created by WordPress by default is : medium_large (768px). This is not listed in media settings.
- Save Changes.
As this is default feature of WordPress you could refer CODEX for extra information.
Settings Media Screen – WordPress Codex : https://codex.wordpress.org/Settings_Media_Screen