Featured Image sizing issues

Post example: http://balanceandwomanhood.com/secrets-to-make-your-marriage-last/?preview=true&preview_id=1079&preview_nonce=c5a73b6e80&post_format=standard

Problem: The featured Image size is HUGE. I’m only using it because without it, I can’t get the post to show a thumbnail picture on the recent posts widget. I would really like to know how I can either:

-Allow the posts to show thumbnail pictures on the recent posts widget OR
-Allow me to decide on what size to use for the featured image

I don’t know any coding (yet) hence the reason for my frustration. I’m sure this would be easier if I did.

Jackie