I bought and downloaded Lightroom yesterday and am struggling. I need to export images (jpegs) for competitions (1920pixels x 1200pixels) and can't work out how to do this. My screen bears no relation to any of the tutorials I've watched and I can't work out how to do this. I seem only able to set one dimension.
It sounds like maybe you are using Lightroom and watching tutorials for Lightroom Classic.
In order to export your image at the specific dimension of 1920 x 1200 you need to first crop your photo to the same proportion. Unfortunately Lightroom doesn't make this very easy because there is no preset for 1920 x 1200. You probably don't want to crop your image anyway. Go back and reread the directions for the competition. It might say to make the longest width no more than 1920px and longest height no more than 1200. It probably doesn't require you to enter your photo at those exact proportions.