Here is a small plugin to render images with a given DPI.
The "Draw" button just draws the image without raytracing (a fast way to see that the height/width look good).
The "Ray" button raytraces first, before saving.
The functionality is also available as a script (see my .pymolrc here).
First the imperial version. The metric version follows.
To install, save the script as e.g. rendering_plugin.py or rendering_plugin_metric.py and install via PyMOL's Plugin --> Manage Plugins --> Install menu.
The plugins are available through the project, Pymol-script-repo.