Theoretically, this should work, but it's not. I am still seeing images of a variety of sizes even though I actually re-uploaded the images. Also, no option in the Media box add image is created (though not sure it should be).
Did that several times but it does not seem to resize to the size specified in the custom sizes I added to functions.php. Also please note these are not the default featured image I am working with. These are simply images uploaded through a new custom field which I named "featured directory image" - though I doubt this should be an issue.