Penis size and the retractile penis
Size is such a big thing for men, though few women really bother about it. (This website has a good review of the size issue, and what women really think about it. You can also see a lot about penile myths and facts here.)
For the record, therefore:
The average size of a non-erect white adult male penis is 3.4 inches long by 3.9 in girth
The average size of a white male's erection is 5.9 inches long by 5.0 inches around
Over 60 percent of men are between 5 and 6.5 inches long when erect
Almost one man in 15 has an erection less than 5 inches long, and almost one man in ten has an erection about 4 and three quarters inches in length.
The penis can vary from one inch when erect to 12 inches erect, though such huge variations of size are unusual.
The smallest flaccid penis is usually the retractile penis, one which has a tendency to retract into the sheath of the foreskin or even the body cavity. Between 1 and 3 % of men have a penis like this.
The photos below show some of the penises which fit this category becoming erect: the obvious point being that though they may be very small when flaccid, they can be average size when erect, though this is not always the case.
Many physical conditions which cause stress can reduce the apparent size of the penis due to the drawing in of the muscles because of adrenalin exposure.
The retractile penis
Smaller than average penis size
In cases like the one below, it's reasonable to assume some developmental abnormality has affected the growth of the penis.
The abnormally large penis