Customer wants 3840 wide and 2160 tall, I put those sizes in but when the photo exports it comes out as 3757 x 2160. How do I get the size to stay as I put it in?
Using Lightroom 5.7, PC.
You have to first crop to an aspect ratio of 3840/2160, or equivalently 16/9, then your export will work properly.
If your images need to be those exact dimensions then you need to crop them to that aspect ratio which is 1:1.7777. Then in the export dialog specify the dimensions of the long edge.