Is my penis too small?

Honestly, penetration doesn't do much for women. It's all about clitoral stimulation which is where all the sensation comes from. We do not orgasm otherwise, so penis size is not that big a deal. 5 inches would be enough for a woman to feel the pressure of penetration and can help build towards an orgasm, and the female vagina is only 6 inches deep while aroused so any bigger then 6 inches and you can end up hitting her cervix which is more or less painful. You should be fine. If size bothers you that long then you should consider losing weight.