First, flaccid should be considered when you can bend your penis in half with little feeling and there is little to no stiffness/rigidness.
Second, any man's flaccid size is going to vary depending on his environment, and there is no getting around it. So most of these people that are giving you 5-8 measurement for their flaccid size are talking about 'max' flaccid size. Their penis could easily be made smaller if you put them in a cold environment or on that would cause their penis to retract closer to the body (playing sports and high adrenaline situations do this).
So speaking from personal experience... my penis while flaccid can range from 2-7 in. long and 3-5 in. thick depending on the environment and hard 7-9 in. long and 5-7 in. thick again depending on the environment and stimulation.
The statements about the penis that is big flaccid not getting much bigger comes from the fact that is probably not 'completely' flaccid or it's at max flaccid, which is right around the corner from being hard.