I was explaining that the largest size image you require is 708px wide this would be for the Featured Image when displayed on a Tablet device in portrait view.
So original image should be 708px wide, the height is up to you, although it would be best to keep them all the same size.
I would suggest 708px x 400px.
700px x 400px would also be fine but the image won’t quite fill the container on portrait tablet.
To fix serve scaled image you will need to set your Media attachment sizes in Dashboard > Settings > Media.
You’re large image can be 420px ( to cover Plus size mobiles)
Medium 300px to cover your desktop blog images.
Once done you will need to use the Regenerate Thumbnails plugin to fix your current images.