A recent issue of S&T that compared 8" dobsonians, commercial variety) had some answers to these and other questions. Basically, take the focuser height, racked in, + 1~1.5 inch, + the radius of the tube diameter, and divide the sum by the mirror f/ratio. Then add 0.5" for planetary use or 1" for photo field light coverage. There are other, more complicated math ways to compute, but the answer is almost always going to fall in between sizes available, so you kinda hafta choose!!
AFA baffles are concerned, I`ve seldom seen them used in newtonians. Never really thought or were told that they are necessary, as in refractors. Can`t even say why?? Anyone else out there know a real answer??