I haven't discussed penis size with a lot of women but clearly most seem to be relatively poor judges of absolute size. Some, however, are pretty good at judging relative size. They know whether the guy they are with is bigger or smaller than the last one, or the average of their experience.
And I knew at least one woman who was very good at estimating absolute size using her hands and fingers as a measuring device. If a woman is interested in penis size, it isn't difficult for her to do this if she knows the width of her palm, the length from the tip of her middle finger to her wrist crease, the circumference encircled by her thumb touching the tip of her index finger or middle finger, etc.
As for length, one woman had a simple system. If she wrapped her hand around it and it covered the whole thing (shaft and glans) it was short. If some stuck out of her hand but was completely hidden by her second hand it was average. If she wrapped both hands around it and some stuck out it was long. For girth, if she collared the center of the shaft with her thumb and index finger and they touched easily it was thin. If her thumb just touched her index finger tip it was average. If there was a gap it was thick (she did have big hands).