The width of the content area is 1170 pixels, so it would be good if your images had a minimum of this width. If they are bigger, they will be proportionally scaled down to fit the content’s width.
There are no restrictions for height. For example, at portfolio, the width of the images is 1170px but the height at many cases varies. Also, at the demo’s blog section, the featured images at the blog posts are 1170 x 520 pixels, but again, there are no restrictions for height.