Custom thumbnail sizes

I am working off of the thumbnails template to create my own template. I need to change the thumbnail sizes. I found information on how to do it here but it is not working for me. I have a folder in my theme folder called cycloneslider. My template is inside that. I have edited the slider.php to show the thumbnail size that I want to use. I also copied the cyclone-slider.php file and changed the thumbnail size on there. I always come up with an error “Notice: Undefined index: 580 200 crop.” That is the size I want for my thumbnails. And, obviously the thumbnails do not show up. As far as I can tell everything is defined and it seems to not be loading my edited cyclone-slider.php file. Any feedback would be very helpful. Thank you.

https://wordpress.org/plugins/cyclone-slider-2/