Store header image size depends on your store template - image width is 1000px and image height is different for each template.
That said, you can set header image size to what ever you need in settings -> design -> custom css -> home page top images
There you can set the height of the top images to what ever you need (assuming that image width is 1000px)
So for example your images are 1000x300px, you can set top image height to 300 (in settings -> design -> custom css -> home page top images) and than upload 1000 X 300px images to the top area of your store, and all of your images will display as they are (not cropped).
Generally you should use images with resolution of 72 and upload images that are at least 1000px wide to your store header.