generally speaking, a penis no longer than the longest direction of a character's hand when erect, and noticeably smaller than half the length of their thigh when erect. Though arbitrary, in extant humanoids in real life, these relative measurements are approaching the limits of the largest common (~95% of the population) healthy penis sizes. If the size when erect is about the size of the character's thumb, it is the size appropriate to the small_penis tag.

when flaccid, this generally means no more than the tip would be visible from the backside an average sized set of balls in even the lowest hanging flaccid penises.

if a range must be given to these sizes, they would be between 10-18cm(4-7 inches) when erect and 5-15cm(2-6 inches) when flaccid. this, however, necessitates a very humanoid body for measurements to be accurate.

as this tag is rather subjective, it should be used most on works that present characters with attainable humanoid proportions. However, auxiliary appendages and features (ex. thick tails, animal shaped penises) are appropriate, so long as they follow the same rulings.