If I select the stacked thumbnail plugin and check the 'keep aspect ratio' box, i only can input a width (what's absolutely right), but all thumbnails seem to get their aspect ratio from the first image, so if i mix a 2:1 image with an 16:9, all thumbnails will displayer in 2:1.
In a thumbnail stack, all thumbnails will and must have same size. The "keep aspect ratio" parameter allow you to set only the width and then the height is computed according to the size of all the panoramas thumbnails. It is computed according to the greatest width and greatest height of all your panoramas. So even if you have many panoramas with 16:9 ratio, if your equirectangulars are wider than the 16:9, then your thumbnails will have a 2:1 ratio.