Based on the recent comic, we now know enough about the bat-schlong to do some maths.
First to establish a reference point. The average American Man’s thumb is 1 inch wide (x). I estimated the shape of the thumb based on the angle of the shot (estimate of shape is the blue outline). His thumb is probably about 29 pixels wide, but could be up to 40 pixels wide based on the angle of the shot.
The bat penis is roughly 130 pixels long from base to tip, assuming a top-down view. We can estimate how big the schlong is in inches by using:
[Were Thumb Length (Inch) = 1]
130/29 = 4.48 inches ; 130/40 = 3.25 inches
Based on this we can assume that the Bat-Schlong, when flaccid, is between 3.25 and 4.48 inches long.
The study does not account for ambient temperature, or which side of the shower/grower boundary Batman falls on, but I would say that 4.5 inches soft is…. at least worth further study.