According to the researchers:

“Women preferred a penis of slightly larger circumference and length for one-time (length = 6.4 inches/16.3 cm, circumference = 5.0 inches/12.7 cm) versus long-term (length = 6.3 inches/16.0 cm, circumference = 4.8 inches/12.2 cm) sexual partners.”

Those values are slightly larger than the average American penis size: roughly 5.6 inches in length and 4.8 inches in circumference when fully erect.