If you're reading this you've probably had a run-in with a container size discrepancy in the past... The most common controversies are over #3 vs. #5. What caliper to expect in a #65 vs. #100, what container size to use for a 5" and 4" pot vs. #1. First off - if you're spec'ing container trees on their caliper, to assure the biggest bang for your buck, use the following chart as a guide:
To spec a caliper higher than what is on the above chart, you run the risk of having girdling roots in the future. This is a costly problem that can sometimes result in death. To spec a caliper smaller than the container size, you're asking your contractor to purchase a tree that is not considered 'ready'. This means when the tree is pulled from the bucket a giant mass of soil will fall out because the roots have probably not made it to the edges of the bucket. Nobody is getting their money's worth.
On shrubs, there are a few common practice standards that seem to be a 'secret' among the contractor and supplier realm. Everyone specs a #5. There are certain species that will sometimes come in an actual #5 container; Rosemary, Agapanthus, Agave, Red Yucca, Soft Leaf Yucca, and Prickly Pear. Five gallon buckets are much deeper and narrower than a three gallon. Three gallon pots are cheaper to manufacture and are typically blow molded as opposed to injection molded. To expect other species in a #5 you're nearly doubling the cost of the materials to gain 15-20% of your desired height/spread spec.
When it comes to perennials and groundcover, 4" is a popular VE (Value-Engeineered) move. However, while 4" pots cost about half as much, they're about 50% less available too. This means the contractor will usually end up purchasing #1 material anyway. Also - in Central Texas we are all very familiar with heat and drought, 4" pots need to be watered extremely frequently as they have very little soil around the roots to keep them alive. One gallon plants fare much better.
Now, back to trees. I have attached this handy chart that I hope gets printed out and pinned up at your desk. It is not perfect, and it certainly does not represent every grower, and every species, but it is a quick, easy guide to spec'ing container grown material. We hope you find it useful and as always, encourage any questions or comments you may have!