TENON won't be of any help. It calculates the DOC needed to cut *regular* polygonal tenons.
Rectangular stock would still require that the diagonal of the stock correspond to the diameter of the round.
a,b = stock dimensions
D = diameter of round
a^2 + b^2 = D^2
So, once you've chosen one of the stock dimensions, a, the maximum size of the other dimension is given by:
b = sqrt(D^2 - a^2)