Ingmar – Setting up Image Sizes
Please notice: Before you upload images you should to set up image size.
Please go to >Settings >Media and define settings for WordPress media uploads:
Thumbnail size: Width 80 – Height 80
Medium size: Max Width 448 – Max Height 9999
Large size: Max Width 738 – Max Height 9999
Don’t forget ‘Save Changes’
Some additional info: https://codex.wordpress.org/Settings_Media_Screen
To apply those changes to already uploaded images you can use plugin:
Force Regenerate Thumbnails allows you to delete all old images size and REALLY regenerate the thumbnails for your image attachments.