If it's too small/unsafe you'll end up with wildlife not using it or being funneled in for other for predators. The larger and more skittish the animals you want to use it, the larger you're going to want to make it. Since they're calling out mountain lions, which are quite skittish, they're going to need a large (and probably partially covered) pathway for them to want to use it--as opposed to treating the freeway like the barrier it is to wildlife.

I'm sure there's more to it than that, but making it desirable, accessible, and for it to feel safe for the target animal population is important for it to accomplish what it's intended to do.

There are smaller culverts all over the place for smaller wildlife. I've even seen some called out in small walking paths--there for the frogs and other amphibians.