Featured Image size for 2 column "slider"

Loving the NewsMag theme so far. However, for the life of us, we cannot figure out what size photo would actually fill the full “featured” photo area when using the “slider” element, across two columns (we have our page split 2/3 + 1/3 there. You an see it at http://www.mercerme.com. We’ve tried a couple of different sizes, including using full sized/4:3 photos from our DSLR. Not matter what we use, there’s a small grey bar at the bottom of the featured image when shown on our homepage in the slider, indicating that it’s being cropped or somehow not big enough.

Any thoughts here on what aspect ratio or size to use would be helpful.

Thanks!