Hi Ali thanks for your question. One of most important point in history you have not mentioned,how old is your marriage.Relax thin sperm or thick sperms ,it is just a myth.nothing else.Chances of your wife getting pregnant depemds upon number of sperms,morphology of sperms,motality of sperms and many more factors which you will come to know once you will get your sperm analysis done.Average size of penis is 4 inches.so size of your penis is not small.Size of penis is also not related to chances of conception.Most of seminal fluid comes out from vagina after sex that is normal and you need out 3 out 4 millions sperms only one sperm to get your wife pregnant.Strong erection and premature ejaculation are two issue which are in your mind.You have to get them out of your mind.Premature ejaculation can be taken care by prolonging sexual act by foreplay before actual sexual act.Consultation with gynaecolgist is advisible for evualvation of infertality. Hope this answers your question.