I favor gigantic maps, but with a bit fewer habitable worlds than the true scoremonsters around here like.
When I have more than five or seven worlds finishing their build queues in a turn, I use the Colonies tab to add more projects to the incomplete worlds and check focus and/or Starport settings for worlds that are finished with tiles for the moment. If things are quieter, I just click the green squares on the main map to do the same thing.
Any rules of thumb on econ worlds versus research or manufactory world. etc.
IMO, the basic rule here is that The Map Is The Game. If a world has major bonus tiles, you have some thinking to do. If it is smaller and has no bonus tiles, you should probably make it a money farm, but you should think twice about that if the world is in an active or possible combat zone. Small, unremarkable worlds off the beaten paths are best used as tax farms.
Large, bonus-rich worlds that are also in the thick of things are likely to be your drama points. They'll help you win a region in some long wars, and make you seriously regret your failure to be more defensive in others when your enemy takes them and uses the infrastructure to slap you hard for your sloppiness.