A penis that is an accurately measured 8 inches in length with a circumference of between 5.75 and 6 inches is not abnormal, just large. 8 by 6 is often referred to as the "Ideal Penis Size".
As far as the range of girths, I would imagine there are some long pencil dicks out there, perhaps with a girth as small as 4 inches, and some cannons with girths as great as 8 inches.
Incidentally the difference in dick volume between the pencil and the cannon is four fold, not just the double the circumference implies. The pencil delivers about 1.25 cubic inches of dick for every inch of length versus the cannon's 5 cubic inches. Four times more dick per inch. Girth Rules.