A Box Newel Post is a square post, usually with cap and trim. The sides of the newels can be flat, fluted, routered, with recessed flat panels or raised panels. You may want to inlay some other materials such as tile, metal or stone. They can be custom made or purchased.
Starting newels average 6 1/4" to 7 1/2", or larger Landing and balcony newels can be smaller, 3 1/2" to 4 1/2".