Can you better explain the distances? Are you saying that the tank is at an elevation 200' above the garden area? or is it the distance you walkfrom the garden area to get to the tank? On the 25' dimension, are you saying that the tank is on a 25' tall stand?
All that matters in terms of static pressure is how high the bottom of the tank is above the garden area (vertical distance). Roughly, you get about 0.5psi for every ft. So, if the tank was 100' above the level of the garden, you should see roughly 50psi. Measuring from the bottom of the tank give minimum head (tank nearly empty). With the tank full, you get additional head from the height of the tank.
For the size of the pipe, it depends on how much water you are looking to flow (gpm). Larger diameter means less pressure drop for a given flow, but larger pipes do cost more $$$.