A bigger line would only help if the current line was being asked to provide more water than it normally could. For most installations, a 1/2" valve will be fine. With one showerhead, it won't make a difference. if you had many, a larger valve would be useful. You may want to switch or adjust the pressure switch so you have higher pressure. If the pipes in the house have galvanized pipe, those could be corroded so they are the size of a pencil inside, and you may have good pressure, but lousy flow to your valve. What is the pressure switch set to?