Size of penis is genetically determined & doesn't increase after a certain limit..
And premature ejaculation
is related to anxiety
about whether you will be able to perform or not..
So...don't worry about that & stay relaxed during intercourse.
It will be better to consult an Urologist
to find out whether there is any organic problem in your penis or genital system.
And also to learn about various methods/techniques to prevent premature ejaculation.
Other than that...performance can be increased by using diet rich in fruits, vegetables, fish, nuts & honey.
Also avoid taking any over the counter drug which claim to increase size of penis or improve performance...they may be harmful.
Wishing you a good health.
Let me know if I need to clarify how further.
Thanks & take care