Eat watermelon. It's not an herb, but it has properties that might help increase the size and duration of erections in a similar way arginine. Watermelon contains an amino acid called citrulline, which gets converted into arginine and leads to the dilation of blood vessels.The fact that watermelon contains citrulline is a relatively new finding, so no studies have been conducted to find out how well it really works or how much watermelon you'd have to eat to see the benefits. However, since watermelon is considered a superfood when it comes to men's health, you can't go wrong by eating plenty of it while it's in season.