I am exporting some pictures that are offline, but with smart previews. I export the maximum available size, but the exported pictures are smaller than 2560 px on the longest side, often less than 2000 px. Why is that? I thought the smart previews always had that size.
You probably have a crop applied to your image. The smart preview is a 2560px max representation of the full-size image, without any cropping. If you're applying a crop, your export from the smart preview will have smaller dimensions, just like if you were exporting using the original.