There is nothing you can do about your size unless you consider something surgically. You need to talk to your doctor about what may be possible there, if you really want to go that route.

I am going to first point you to an excellent series of articles about this topic from The Mayo Clinic.  They say that there could be many causes for this including psychological causes and/or medical or physical causes.

Some of the medical reasons one might have this issue include:

• Abnormal hormone levels
• Abnormal levels of brain chemicals called neurotransmitters
• Abnormal reflex activity of the ejaculatory system
• Certain thyroid problems
• Inflammation and infection of the prostate or urethra

I know it may seem embarrassing but it may be wise to seek advice from your doctor on this to rule out any sort of medical issue. 

The Mayo Clinic article suggests treatments ranging from the use of topical creams to taking Prozac.

Paula Hall, a psychosexual therapist gives these tips:

• Exercise your pelvic floor. Many men say they are able to delay ejaculation by squeezing or pushing their pelvic floor muscles.

• Stop and start. This is the most common technique used in psychosexual therapy. Practise stimulating yourself to the point just before ejaculation, then stop. Start again when the sensations have subsided. Repeat this three times. You should find the length of time before each stop gets gradually longer.

• Change strokes. If you feel you've gained more control with the stop-and-start technique, try changing your stroke to something less stimulating rather than stopping all together.

• Change positions. During intercourse, try the stop-and-start technique when the sensations become strong. When you're more confident, try changing positions rather than stopping.

This is all we have to give you about this. Talk to your doctor to see if there anything they can provide.