StudioPress recommends using the Regenerate Thumbnails plugin – https://wordpress.org/plugins/regenerate-thumbnails/ for the Genesis Slider whenever you change the size of your images in the Genesis Slider Settings…..I use it myself for the slider as well as any other part of my site that deals with Image Thumbnails.
So pick the size you want first, and then run the Regenerate Thumbnails plugin and it will automatically adjust your images to the size you have specified in your settings.
If that doesn’t work (it should) I can login for you and figure out exactly what’s going on. If you want me to do that, then you can create a temporary user/password and I can take a quick look.
Let me know if this works.