All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Basically uncovering one's private parts to other than one's spouse is not allowed unless there is dire necessity.
Therefore, if your penis size is too small to satisfy your wife's sexual desire then you are allowed to undergo a surgery or take medicine to recover the normal size. We have said in our Fatwa 85857 "If the size of your sexual organ is so small, and cannot function as usual then you may undergo a surgery to correct it. If the size is normal and you can fulfill the desire of your wife then having surgery is forbidden since uncovering one's 'Awrah without legal (Islamic) justification is forbidden. In addition, it is changing in the creation of Allaah by modifying it."
Also see Fatwa 85994.
Allaah Knows best.