Export a specific size jpeg

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. 

Yes, difficult or impossible to do easily in Lightroom (Cloud version 3.2).

eg. if your image is cropped square- an export with a long edge would export at 1920x1920 which exceeds your comp specified size limits.

(Lightroom-Classic does allow an export for a photo to 'Fit' within stated 'dimensions' eg.w1920 x h1200 irrespective of the photo's format ratio and orientation.)

If you have Photoshop (or other editing software) installed, it may be easier to export full-size and do the resize in Photoshop.

Which version of Lightroom are you using? (Help - System Info)