What diameter of Metal Gate posts are needed for a 10' x 4' tubal gate
Any steel post of 3 or 4" (2" if it is a light weight gate) sunk in the ground 3 ft with concrete poured around will hold a steel gate and not bend. However, in my experience the real problem is, depending on your soil type and moisture, the weight will gradually pull the post over.  Two solutions:  1) On the hinge side use two posts 4ft apart with cross bracing.  2) if the area is reasonably level, buy an wheel to put on the free end.  It will take all the weight save you a lot of potential problems down the road.