What are you looking for, best practices guidlines or other companies specs?
There is a growing consensus that smaller trees are better then large trees in the long run, due to the greater stress from loss of root system that larger trees have in transplanting.
Bassuck of cornell has done some studies that show bare root of 1-1.5 caplier inch trees is less labor intensive, thus mor cost effective. They also retain more root system, so have a greater vitality when transplanted.
Zilmer at Midstate Tech in WI, and others did reaserch that shows poor planting leads to stem girdling roots. Johnson and Haur of UMinn builed on that with statistical data to sho that a below grade basal flair will cause encircling roots to form and give a very high probabbility of early mortality by girdling root.
American Nurserymans Standards state minimum root ball size should be 1 foor for every caliper inch. This is a bare minimum, and some trees like oaks should exceed that greatly. On important trees one should use the largest root ball that is feasable to transport.
Zilmer devised a method of plantingf whit B&B if it is the only thing available, where the root ball is excavated and eicircling roots unwouldn and burrid in radial slit trenches.