There are 4 penile erection stages. They are:
Not-erected - When one is totally not aroused
Half-erected - When one is aroused
Half-half - The state when one's penis is starting to get
erected(usually a couple of minutes) and when a person had just
ejaculated.
Full-erection - This is the longest the individual's penis can go.
It usually happens only when a person has been 'in contact' with
the penis for a good amount of time without having an orgasm.
EmbarrassingProblems.com
I find these quotes to have helpful information.
Quotes from Go ask Alice
Almost every male seems to envy someone else's penis. He wants one
that's longer, wider, harder, with more staying power, and he
assumes that some other man, or lots of other men, have one just
like that. The truth is, there is less variation among hard penises
than among soft ones because a smaller soft penis will increase
more in size during erection than a larger soft penis.
Men invariably assume that their partner will prefer a bigger
penis. The reality is that your partner would most probably prefer
a more skilled, caring, warm lover. Maybe you guys could stop
focusing on your penis sizes, and instead work on developing your
skills and exchanging tips on touching, kissing, caressing and
lovemaking. What do you think?
Alice
To all the men who question their penis size: While it is true that
we find well hung penises to be visually pleasing, size really
matters less when it comes to sex. I personally would rather screw
a man with a 4-inch penis than a man with an 11-inch penis if the
4-inch man knows how to use it. My experience has been that
extremely well hung men just want to "jam it in" for their own
pleasure. Four to 6 inches is just fine, as long as you take time
to make love to us.
Gabbie
This response is for all those guys out there with small penises.
My partner is kinda small... BUT this guy can really turn me on. He
kisses and touches me so passionately that it drives me wild. There
are sexual positions that work better than others, too. Doing it
from behind allows for better and deeper penetration and in my
opinion, a larger penis can actually hurt. So give it a go and
remember all the in between stuff first... it's the most
important.
Sincerely,
Loves a small one[/quote]
"It's not the size of the ship, but the motion of the ocean."
Don't let the size affect you! 
