First of all, ANY belief like this is a limiting belief and should be banished ASAP, along with all the others "I'm too poor / short / overweight / bald" etc.
Here are some FACTS
1. The average size is 6inches. This means that roughly 50% of men are less, and roughly 50% are more. So congrats you are in the same category as about 1.6 BILLION men.
2. Women's internal muscles expand to fit around everything, from your finger to a fucking baby. Unless they've been massively stretched out they will feel it and enjoy it.
3. Every girl will have slept with a bunch of different guys - tall, short, muscly, skinny, black, white, etc. You're overexaggerating the importance of one factor that they care about as much as the size of your right deltoid, your car's engine, or your chequing account. Yes it's a factor that can influence her to be more or less attracted to you, but it's just one of MANY and most girls care WAY LESS than you think about +/- a few inches here or there. Sure it's NICE but it's by no means everything.
Go out and game, be confident and happy, apply your favourite techniques and routines, and you will see results, get laid and realise how this was never a big deal.
However, in the spirit of a total and complete commitment to self-improvement in all aspects of life, here is a set of exercises, if you still feel you need to do them, that I guarantee through my own experience do actually work, not only for increasing size but also (and MUCH more importantly!) stamina.
http://www.lpsg.org/95782-free-classic- ... utine.html