At https://developers.cloudflare.com/images/transform-images/#format-limitations I can see there are limitations to what can be done with Image Resizing, specifically 12k pixels wide or high and 100M total pixels. However, it's a little unclear if this refers to the the original image or the thumbnail being created and when I experiment I seem to be able to resize images more than 12k pixels tall without an issue.
Transformations let you optimize and manipulate images stored outside of the Cloudflare Images product. Transformed images are served from one of your zones on Cloudflare.