I want to ask what is the best natural way to have a bigger penis and better stamina in having sex?
Why? If the problem is that she's not satisfied, neither of those are the solution. The G-spot is only about 1-2 inches into the vagina. A 3-inch penis is quite sufficient and a longer one won't help at all. Slowing down will help somewhat, but your fundamental problem is not that your tool is too small, it's that it's the wrong tool for the job entirely. Manual or oral stimulation of the clitoris is much better method of satisfying a woman, regardless of penis size.