There is no law against "open carry" but then again there's nothing supportive of it either, which makes it legal to "open carry" in the Kansas City, Mo. area. There are areas however where "open carry" is not permitted. Open carry is not permitted anywhere North of the river, or anywhere near Downtown Kansas City, Mo. (where the buildings are) Other than that, the only other areas I know of are down in Belton, Mo.
As for the size of weapon, while Open carry is not restrictive of any firearms that I know of, the "open carry" thing usually is with handguns only as rifles tend to create too big a signature and IMO, they are not "defensive" weapons. And they tend to attract too much attention from the Police.