Hi @jakobdk,

I hope you are well today and thank you for your question.

There are various sizes of featured images based on the design as listed below.

'gillion-portrait' 	 372 X 484
'gillion-square'   	 585 X 585
'gillion-square-small'   220 X 220
'gillion-square-micro'   80 X 80
'gillion-landscape-small'420 X 265
'gillion-landscape-large'1200 X 675
'gillion-masonry'        585 X 1170

We recommend you to set highest size of it that is 1200 X 1170 then regenerate thumbnails using following plugin so that all other image sizes will be automatically created.

https://wordpress.org/plugins/force-regenerate-thumbnails/

Best regards,
Shufflehound team