[Support request] Setting Featured Image size

I’m loving the functionality of this, seems to be just what I’m looking for. My question is, when setting an image dimension of 350×200 for post images, that also sets the Feature Image to the same size. Obviously it needs to be bigger. I’m setting the image size to control the aspect ration of post images so that they display neatly.
How to I control that and also have a large featured image?

Many thanks,

Neill W