Hi Josh,
Excellent question. The images in the hero section will be cropped a bit. Since the hero image will always completely fill the hero area and the browser window can be any size there will generally be some degree of cropping. The only alternative would be to skew the image as the browser window changes size and that would look odd.
Generally what I suggest is having a bit of a buffer in the image that you choose between the main focus of the image and the edge of the image. You want to have that buffer so that the important parts of the image aren’t cropped out as the screen changes.
For sizing, it is flexible, but I generally shoot for images that are around 2000 pixels wide by 1200 pixels tall. That seems to yield a good balance in image size/quality and matches commonly used browser window aspect ratios.
If I can help out, let me know,
Bill