I guess in turns of room it'd depend on whether Kaido has higher haki than law.
Seeing Haki focused on one part of a body like fist or foot, and seeing Haki covers the body, I look at Haki as like honey. You put honey on one small area of a large dish/surface. It keeps its thickness.
You then need to cover the surface with honey, you may pour some water to lighten the thickness of a honey, so it will spread and cover certain surface completely.
Pica and Vergo covered themselves in Armament Haki while people like Zoro and Law (specially Law), they focused their Haki on fist and sword. I believe Law did that same as Zoro.
I believe that Haki its armor, its invisible armor isn't strong once it covers all the body, but strong enough when focused all on fist, forehead or foot.
I believe Luffy focused his Armament Haki on the side of his abdomen when Doffy did his Athlete Kick, and Doffy says: It's still rubbery.