For our service conductors, overhead and underground both, we used triplex and quadruplex with a neutral two sizes smaller than the phase conductors. 3/0 had a 1/0 neutral, 4/0 had a 2/0 neutral, etc. We didn't "size" neutrals as such. We chose the appropriate phase conductor size, and the neutral just came along as part of the assembly, two sizes smaller.
As far as what we used to determine conductor ampacities in various conditions of service, it was - as iwire alluded to - a mish mash of NEC Tables, manufacturers' listings, our Standards departments best guesses, and probably most often, the engineer's best guesses based on years of experience. Despite the lack of one all-encompassing chart or table, the system worked pretty well, striking the right balance among safety, economics, and customer satisfaction.