I haven't tried it, but if you set the shadow map limits in your Preferred State PZ3, it should be applied to every new scene you start. Did you re-install Poser when you upgraded your OS? That may be the reason it's changed. As far as I know, 1024 is the default limit (and 256 the default map size), and has been since Poser 3. However the limit is an arbitrary one and really only depends on how much RAM you have.