Featured Image Size

Hello, A quick query. Is there a way to change the featured image size that displays in the 'Featured Content Display' by default it resizes these to 350 X 197 and selects the center portion of the image. I would like a square image to display instead (350 X 350) Is there a way to do this with custom CSS? Dev site URL is: http://dev.7thwaveweb.com/kingsarms/ Many thanks