. . . design the ductwork and that will determine what size (CFM) blower is required.
Mostly correct- but first comes the WW machines- each has its own minimum CFM requirement. That data is available from various sources like Bill Pentz's website, though he didn't originate it (I don't remember the source but it was some professional association). The data also often implies larger dust ports than most hobby WW machines come equipped with. So you'll need to enlarge the ports or adjust the computation.
Then you can design the ducting that will move the CFM needed by the largest machine and/or the one located the farthest from the blower- the worst case. With hobby woodshops where only one machine is in use at a time, there is no need to use step-down ducting like in commercial shops with multiple machines running at a time. Run the same large duct (and flex) from blower to machine. Don't even step down at the drop or blast gate. Also, if you have multiple dromps you will need to put a blast gate
at each machine so only one branch/duct is in use at a time. Otherwise, like industrial installations you'll need a ginormous motor/blower. Also, if you sealed all joints properly, have blast gates that seal well,, and don't have leaks, you won't need blast gates where branch ducting joins the main duct (where they can't be reached.)
Unfortunately, you'll be somewhat limited by the diameter of ducting available, especially in PVC- for anything but a tiny shop you will need to use 6" duct. (I won't go into the math, but 4" duct won't permit the CFM needed for anything but the smallest machine. You can get larger diameter duct in metal, but at a really high price. Don't forget to add the SP generated by all elbows, fittings, diameter and length of ductwork, etc. and don't forget the cyclone and filter(s) if using them, when you add up everything to compute CFM needed.
Only then you can select a motor/blower unit. Unfortunately, somewhere along the way, reality and the size of your wallet strike. Then you do the best you can do. If at all possible, the best approach is to vent outside with no cyclone and no filter(s). Hobby woodworkers are about the only folks who attempt to recycle dust collection air. Even with their huge cyclones and bag houses, commercial shops do not try to do that. It is too expensive and puts too much restriction on CFM. Remember a dust collector is a "system" it doesn't matter where the cyclone or filters are located. They can be in front of the blower or after it, but they still add resistance and increase SP.