Hello there....., The typical erect penis is usually 5 to 6 inches long with a circumference of 4 to 5 inches. There's more variation in the size of flaccid penises. Some guys are genuinely smaller than that. In rare cases, genetics and hormone problems cause a condition called micropenis -- an erect penis of under 3 inches.... Losing weight will reveal more of that hidden shaft that's buried beneath belly fat. It doesn't actually increase your size, but it will look that way..