along their long edge with the cross-platform XnView MP: Tools > Batch convert... > Actions > Add action > Image > Resize > Mode > Longest side. (In Lightroom: File > Export > Image Sizing > Resize to Fit: > Long Edge.)
UPDATE: I had completely forgotten about the tiny and handy Resize 2.6, which allows batch resizing by percentage, height, width, or maximum dimension while maintaining aspect ratio; it's been listed on the graphics page for ages.
/misc | May 10, 2019