I am trying to setup an Axes Aligned Bounding Box and the class Aabb has the following function.

void Ogre::Aabb::setExtents(const Vector3& min, const Vector3& max);

What I'd like to understand is, given a geometry that I read in, how would I calculate the extents of the geometry so that I can call this function?

Run through every vertex of your object and find the minimum and maximum position. E.g. smallest x,y,z and highest x,y,z.