Average girth is 4.6". In my opinion anything over 5.0" is thick or at least thicker than average... about 30% are 5.0" or thicker. 5.5" and over is unquestionably thick... less than 8% of guys are this thick or thicker. 6.0" and thicker is extremely thick... less than 1% are this thick or thicker.
Now the tougher question is where you measure this erect girth. Some say mid-shaft only. Some say anywhere below the head picking where it's thickest. Some say the base. This is probably because some are thickest in the middle, some are thickest near the head, and some are thickest near the base. Some are nearly the same thickness all along the shaft.