Is it possible to set an Export preset - so that the SMALLER of an image's two dimensions is fixed?
As an example, I can create a preset to give me dimensions of, say, 1000 x 600 pixels. Or, 1000 x 542 pixels, and so forth. The image will adjust as needed. Can I go in the other direction? I would like to set 1000 pixels as the default on the smaller side, with the image adjusting on the larger side to keep the proper aspect ratio.
Just make the larger dimension in the preset much larger than you'll ever need and Aperture will basically ignore it. It will not change the aspect ratio of the image.
For example, if you have a 5000 x 5000 image and you export it with a preset that is 10000 x 1000, the resulting output will still be 1000 x 1000. Basically the export preset never crops or distorts your image.