In the Develop Module, click on a dotted rectangle (Crop Overlay) and then on the "Original." In the drop-down menu, click on the "Enter Custom" where you will enter the required aspect ratio. The aspect ratio could be as simple as 2:3, or 4:3 (in which case you would not need a custom ratio size, since LR already has the most common ones), or as complex as, for instance, 878x453 (say that is the exact dimension in pixels the client required). Now you have a custom (or standard) ratio size you need to apply to every image in the series.
This is not the end, however. In the File>Export dialog, you will then have to enter the exact pixel dimensions in the Image Sizing field.