What makes for a “thick” penis is a matter of opinion. But it’s a great question because it’s clear that women prefer more thickness over long length. I’ve aggregated a number of credible studies on penis length for work and have some info on what women think is the “ideal” thickness and when it is on the side of too thick.
These figures are penis girth, or circumference, around the middle part of the shaft. Ideal fell in a range between 5–5.5″ (12.7–14cm). Women said they prefer thicker penises, so I suppose 5″ is the low end of “thick.” The “too thick” numbers varied a lot, but pretty much seems over 6.25″ (15.9cm) and we start getting into the too thick range for a lot of women.
Penis length seems to get all the attention, but when you survey women, it’s clear that girth makes the difference as long as the guy has good length. Makes sense because thicker penises expand the vaginal canal more and expose loads more nerve endings that are otherwise up in folds not stimulated by a thinner penis. A penis longer than 6–7″ is likely to hit her cervix, which can be painful. So it also makes sense that women say the average ideal length is 6.4″ (16.3cm).
I write all this knowing full well that these are averages. An individual woman may like smaller or larger than these averages.