The normal penis size is 5.1 to 5.9 inches. There have been techniques which are used to increase the size of the penis like jelqing, penis stretching exercises as stretching makes cells composing your penis grow increasing the length of your penis and kegel exercises which target the pubococcygeus muscle and thus helps in maintaining the erection for a longer time. You should go ahead with these exercises only after a consultation with your urologist as they have their own side effects. The only useful methods are penis enlargement surgery and inflatable implants. But they are also not without side effects which concern mostly with side effects of surgery.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.