What size did you want your images to be? You should be able to upload whatever size you want (as long as it fits inside of the content area width).
As you may already know, you can select the Featured Image size in your Genesis Theme Settings page. If you need to add/change a size, then you can do so from your functions.php file. Scroll until you see the Featured Images code and make your adjustments from there.
Once you change your image size, run the Regenerate Thumbnails plugin so it can adjust the size of all your images.