Besides the fact that you will have to make this change each time there is an update to Zenphoto, it will not do what you have asked.
Note that the values are max width and height. So all enabling this will do is prevent larger images from being uploaded. Of course you could find where these items are referenced and change the code to check for exact matches.
But if I understand correctly, all you want is for the downloaded image size to be 1900x1200. It would be possible to make a plugin for the download that "cached" the image at 1900x1200 and provided the download from that cached image. For images that are already that size, no processing will occur. For ones that are not, then Zenphoto will resize them.
Certainly that would be a much more convenient solution.