14" diameter will yield about a 10 x 10 cant, if the log is straight, less if the log is slightly crook.
Walnut has a distinct band of white sapwood, the sapwood band is wider in trees that are fast growing such as yard tree walnut. All in all your most likely looking at a 8 x 8 cant of heartwood, since sapwood walnut is undesirable to most wood workers, it's really not worth wasting time on. Depending on how centered the pith is (how straight the tree grew when it was a sapling) you may be able to get 7 4/4 x 8" wide boards out of the 8x8 heartwood cant. But in my experience, I would not expect anymore than 5 or 6 4/4 x 8 usable boards. Pith ridden boards are not good for wood working either since they are highly likely to split in or close to the pith. You really can't do much with a 8" wide pithy board, if you rip the pith out of it you would end up with 3" wide boards which are not desirable either.
A 14" diameter black walnut is not what I would consider mature, nor would I bother putting it on the saw mill. It may be good for firewood or crazy spinners. But if it were me, I would leave it on the stump to grow for another 30 or so years, for the next guy to saw.
Now when your upwards of 24+" diameter forest grown (not yard grown) then your starting to talk more mature and have something to consider milling into lumber.
Just my thoughts... take it for what it's worth - which is really not much...
