I suggest that you try and run some some small parameter studies to get a feel for how large a shell/plate/membrane area element you can get away with while still maintaining 'accurate enough' results for your application. Of course a 1mm X 1mm mesh would yield better results than a course mesh, but it would likely take too long to run, and you would have a lot of output to deal with. Also, keep in mind that the "right" meshing would also ensure that areas are divided/meshed such that there are areas where you can assign your needed area loads (for a large piece of equipment, for example), and joints are in place where you need to connect other frames or areas, or where you need to assign joint loads, restraints, or links.
