Small size means any penis that is less than 2 inches when erect. if your penis is more than this, it is NOT small. Further for persons with less than average length of erect penis, normal missionary position may not help to achieve penetration.
Next Steps
You must try a position where your partner is at the edge of the bed, and then ask her to take her legs upon herself. You will stand beside the bed and try sex in standing position. This position allows exclusive contact between the penis and vagina and here the lady partners inadequate leg parting doesnot come in way of the penetration
Health Tips
For good erections, one must have sex on empty stomach and also have it in morning when ever you have erections on waking from sleep. For premature ejaculation you will need to consult a sexologist in your area as it involves learning a specific technique which delays ejaculaiton