Specs on your plans drive the materials that end up on your project. Here's a simple tip that some use and some do not, to assure you're getting what you want, and need. Determine the most important spec. Is it height? Spread? Or is it simply caliper? Between species, then supplier the other specs will vary greatly. For example, while you can realistically expect a 6' spread on a 4" Live Oak, you can't apply that same logic to other species... a 4" Multi-Trunk Vitex will have a spread of 10'+, and a 4" caliper Bald Cypress will typically be closer to 5', or less. The most important spec is driven by application... Are you meeting mitigation requirements (caliper)? Are you trying to screen electrical panels (spread + height)? Are you going for height to hide an eyesore on a neighboring property (spread + height)?
Do a few minutes of research to determine the other contributing specs associated with the most important, or 'primary' spec. A quick and simple solution would be to do a search on your desired species on localplantsource.com, click the more options tab at the top of the search results page and filter by your primary spec. Let's say in this example it is height. From a quick glance at the newly refined search results one could deduce that to achieve a 4" Caliper Live oak, you could expect the height to be somewhere between 12' and 14', the spread to be somewhere between 5-7' and the container will most likely be a #100, but could be a #65.
Why is this important? With conflicting specs, you're putting your contractor in a compromising position. The contractor needs to be price competitive to get the work. Obviously, to be price competitively the smaller the spec, the more competitive the contractor will be. Typically contractors have to try and determine on their own, the architect's 'primary spec' and price from there. This will result in nobody getting what they want and again, everyone loses at some level.
Solutions: Call your contractor of choice and ask for recommendations, visit LocalPlantSource.com and run a few quick searches or even call/e-mail our staff, call a few growers or suppliers and get their input on the matter. This activity should take less than ten minutes of your time per species and will save you hundreds, if not thousands of minutes throughout the rest of the project.