Is a 18 inch penis too big for a 16 year old your girl friend says it is?

No male has an 18-inch penis no matter how the male measures it! The average size is 5 and 1/2 inches. No female would be able to accommodate a penis much longer than 6 to 6.5 inches.

I highly doubt you have a 18 inch penis and yes, it would be to long for any female's vagina.