What web site are you uploading to?
Some web sites allow you to set an option to prevent downloading, although that doesn't prevent screen captures. In general, just about anything you put on the web can be copied, so you would have to shrink the image down to prevent someone from doing meaningful things with it. I point out the file size is not the relevant area to shrink down; the thing you want to shrink down is the number of pixels, and this can be done in the Lightroom Export dialog box, or in the Lightroom Web module.