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I recently came across the CNLA (Canadian equivalent of ANLA) standards for nursery stock, and will admit that I find them over the top and overly broad in their categories.
Example: By CNLA standards a white spruce is a 'tall spreading' evergreen. A 5 foot tree needs a 60 cm (24 inch) root ball if field grown and root pruned. A woodlot collected tree needs a 90 cm (36 inch) root ball.
My experience during the past year is that a 5 foot woodlot spruce has good survival rates (95+%) if moved when dormant with only 12-16 inch root disk that is about 4-6" thick.
I have also grown 4 foot pines, 5 foot poplar 6 foot larch, and 7 foot birch in two gallon pots. (#2 Stuewe Tree pot)
This experience makes me sceptical about the standards.
I would expect that standards would be much more dependent on tree species (or genus) E.g. Spruce have a spreading system root system, so the root ball only needs to thick enough to not break. Bur oak has a tap root, so you want grow it in a chunk of sewer pipe. (One with anti root spiraling ridges...) All the trees above have calliper under an inch. (My clients are mostly acreage owners. One criteria is to get the top of the tree out of reach of the deer. Height is WAY more important than fullness.)
I would also expect that a huge amount depends on how the tree was raised, what the climate is like where you plant, what the soil is like for field grown trees, whether it was sprinker or drip irrigated, how often it was root pruned, what time of year the transplant is to take place and so on.
I'm not sure if 'standard' has much of any meaning for root ball size except possibly that no matter how badly you do things, if you have a dirt ball 'this big' you can't screw up.
Of course it also means that you have a bigger product that is difficult (and expensive) to move and plant, putting it beyond the capability of a DIY customer.
What are other people's experience with the CNLA and ANLA nursery stock standards? Or am I mis-understanding the standard? | 
