Just developing that line a wee bit farther.
Often I do not want to warp or squeeze an image but, upon export, I would like to have the facility to specify a width and a height dimension and have LR choose whichever was critical, adjusting the other to maintain proportions.
For example, many club competitions want PDI files that suit the projector resolution, e.g. max 1400 pixels wide and max 1050 pixels high. I never crop my images to suit those proportions - my proportions could be anything from letterbox landscape to postbox portrait. If there proportions are extreme in one direction or the other, I can visually assess whether I am going to specify the long edge or the short edge pixels on export but, for those that are round about the "normal" proportions, it would be useful to specify 1400 x 1050 pixels and allow Lightroom to chose which of those to apply, adjusting the other to suit. There is nothing worse than exporting an image to discover that it is 1400 x 1090 or 1420 x 1050 and have to go through the export process again to correct it.