I work with a lot of panoramas and when exporting would like to set the vertical dimension to a maximum size while keeping the aspect ratio. This would result in a very large horizontal dimension in comparison to the vertical dimension. I can then scroll the panorama at full height to check for detail.
For panoramas, mostly you do not know the horizontal dimension so setting a maximum for the vertical dimension is ideal. I do not know if this is also relevant for the opposite, say for vertical panoramas but if an option is provided for the vertical dimension then it should be a simple matter of doing the same for the horizontal dimension.