There must be a formula to help determine when a particular size pipe would become ineffective and when a specific size becomes ideal (I would think)
Okay, here's the formula: FPM=CFM/Area. You need a minimum airspeed (FPM) of about 2800 FPM to keep most sawdust and chips moving in a horizontal run. For typical dust/chips, you need 4000 FPM for a vertical duct (you're fighting gravity); with large chips you might even need 4500 FPM.
So, let's say your DC can move 1000 CFM. The area of a 7" duct is ~38.5 in^2, or 0.27 ft^2. The airspeed is 1000/0.27= 3700 FPM, which is fine for horizontal runs, but might be a bit low for vertical pick up. That's why you often need to have a short run with a smaller pipe for dust pickup, to increase the CFM in that section. Once the air in that smaller pipe enters a larger pipe, its speed will slow down according to the difference in pipe size (using the formula).
The hard part is determining how much each part of your DC system affects air flow (CFM). I'm not aware of any formula which is able to figure out all the factors- duct size, length, smoothness, transitions, curvature, etc., all of which affect air turbulence and resistance to air flow (not to mention filter resistance)- to calculate the specific impact of individual components on performance. So, the best us mere mortals can do is estimate an expected airflow, and use large ducts where possible (as long as adequate air speed is maintained for good dust movement), and go to smaller ducts only when needed to increase the airspeed for good dust pickup. All of this has to be modified by other factors, such as cost and availability of ducts, shop layout, size and accessibility of individual tool ports, etc.