Resize small featured images for Sparkling WordPress themes to fit content area?

I have some featured images that are a bit small, and when I view on my desktop pc, they do not fill the full width above my post. Is there any way to have it automatically resize them to fill that area?

You can try to add this code to Appearance >> Theme Options >> Other >> Custom CSS

.single-featured {
  width: 100%;

This will make featured images fit content area width but I would strongly recommend to use a bigger images in the first place because this solution is not ideal. Some images might look pixelated because of their small size that will now be zoomed to fit.

Thanks, this was exactly what I needed!
Some of my featured images don’t have larger versions available, so this looks like the best way to go for them.