There are lots of videos and articles "out there" on system design. There are computer models (free) that help you calculate the resistance within a given size pipe while factoring in things like elbows etc to come up with a reasonably close estimate of how many cfm you will experience at various drops. You need to factor both volume and velocity into your calculations.

Another thing to keep in mind is "what machine is going where?" If the furthest drop in the system is for your stand-mounted scroll saw, you can get by with a lot lower cfm at that station than if, for instance, it was serving your planer (while cutting live oak).

If all of your drops are eventually going to end up with a flex hose connected to a 4" port on a machine, then there's no point in having your blast gate any bigger than that port (installed in the smallest line in the run). The most resistance will be in the smallest diameter flex hose, and the design of those is important too. The "cheapies" are ribbed on both the inside and the outside (like dryer vent hose) and have the highest resistance and tendency to clog. The good ones are a lot smoother on the inside (and thereby heavier and more expensive).

The prices of those blast gates rises rapidly as size increases. Probably the only time it makes sense to use bigger blast gate is if the "last thing on the branch" is a wye that has two pickup lines connected to the same tool (like some drum sanders, table saws, and some band saws do). Even then, you may find that two 4" blast gates are cheaper than a single 6" or 8" and can almost always be located in a more easily accessible location.

Don't forget about grounding the system by bridging with bare copper wire anywhere you have flex hose between machine and piping or between two sections of piping. Static is a bad thing.