It is interesting how easy we might fail when estimating the size of an animal without appropriate metric reference objects. I experienced it myself when I stroke the Heck bull at the Tierpark Haag and thought that one might be around 150cm tall at the shoulders. I measured him and he turned out to be mere 140cm, the typical size for Heck bulls. It gets even more difficult when we encounter the animal moving in the field, and often they get overestimated. That’s why sometimes crocodiles or komodo dragons are believed to have astounding lengths like eight or six metres, but once shot and measured they turn out to be within the normal size range of their species. But I was surprised that even guessing the size of an animal next to a person on a photo can be rather difficult. I realized that when had a look at this photo of a Heck bull standing next to a man (http://www.morgenweb.de/polopoly_fs/1.2091254.1423066752!/image/image.jpg_gen/derivatives/galerie_940q/image.jpg). The bull looks large and massive, and I estimated it might be well over 145 cm at the shoulders. Then I did calculations assuming that the man next to the bull is 175 and 180cm tall, respectively. And the result was that this bull is 135 cm tall at best.
I overlaid Wisent and Aurochs at the exact same shoulder height:
The aurochs seems to be the slightly larger animal, which is not surprising since the Wisent is has a short and high body. The largest aurochs bulls therefore were likely heavier than the largest Wisent bulls.
The last one is a comparison of aurochs of three different sizes: 180 cm, 170 cm and 160 cm.