I would have to agree with Kudu, I have seen mixed herds of the two subspecies and the woods are significantly larger than the plains, in both the bulls and the cows. Another means of telling them apart is the woods bison have a fairly square hump compared to the plains, whose hump is more rounded.
My only viewings of Wood Bison, were when they were exhibited at the San Diego Zoo. They did appear to be somewhat bulkier than the average North American Plains Bison.
