What is your pump pressure switch setup for? 20/40, 30/50, or higher? You should have a pressure gauge near the pump and possibly around your filter to determine its state of health. Don't confuse pressure with volume. Most showerheads seem to be rated at a minimum of 40#. If it is a single showerhead, 1/2" should be fine. Not all showerheads are created equal! Based on federal codes, it shouldn't use more than 2.5gpm. That's pretty simple to measure.