I see your point, but that won't help much IMHO. It might be useful for the y-dimension of an object, but for normal collision detection, you want to extend the ellipsoid into x- and z-direction to some degree. You usually don't want to use a tight fitting ellipsoid. Apart from that, it's not a trivial problem and actually unsolvable in a lot of cases. In your case, you won't use an ellipsoid that covers both feet of your hero, because that would extend into the ground.

Use Mesh.getBoundingBox() for now, subtract the min from the max values, divide them by 2 and you have your basic values for the ellipsoid's radius. Add something to x and z just to be sure. Place it above the ground, so that it's close to it but doesn't intersect.