If you set the export size to “Longest Side,” that side will be preserved.
With the “Fit” option, however, you define the maximum lengths of the longer and shorter sides. This means that a side can be shorter than the selected length.
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ADDENDUM
If you want to export images for online presentations, take a look at
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I’ve been using the Nik Collection for a long time and also use the Output Sharpener (w/ different settings) for printing, while others use Topaz AI, for example.