You can calculate the side lengths of the oriented bounding box as std::abs(obb.max_pt.x-obb.min_pt.x) and std::abs(obb.max_pt.y-obb.min_pt.y) and std::abs(obb.max_pt.z-obb.min_pt.z).
std::abs(obb.max_pt.x-obb.min_pt.x)
std::abs(obb.max_pt.y-obb.min_pt.y)
std::abs(obb.max_pt.z-obb.min_pt.z)