1. What size motor (either in watts or horsepower) do you intend to drive the fan?
You will need something enormous for 6" tubing. The amount of dust or chippings the system can carry per cubic foot or cubic metre of moving air depends on the velocity of the air, not the diameter of the pipe. Consider: a sea breeze moves enormous volumes of air, but hardly lifts anything more than a few crisp packets. On the other hand, a tornado in Wyoming really does lift vehicles and even buildings, even though it's far more localised. The significant difference is air velocity.
One man using one machine at a time is quite unlikely to generate enough chips quickly enough to need 6" piping. OK, if that machine is a spindle moulder or possibly a P/T taking thick passes you might get to the point where it's advantageous, occasionally. And if you have several people and machines in constant use, it's a different matter. But those scenarios would need a different class of system, with quite expensive hardware powering it and collecting the "output".
At a maximum I would consider 4" and probably smaller still. If you have a wide-bore, underpowered system (emphasis on 'underpowered' for the diameter), the heavier stuff will just drop out at points where the air movement is slowest (eddies on corners or whatever), and stay there. This may or may not clog things up, depending where the build up occurs.
2. Static build up: you need to earth the INSIDE of the tubing as well as the outside. The outside will otherwise attract airborne dust because of the capacitative effect. Sparks jumping between metalwork pieces inside the tubing or off rounded surfaces inside will potentially cause an explosion. Metal tubing is safe. Insulating plastic tubing -- some of it is high resistance but coducting, which is a lot safer -- would be dangerous. Even my little Makita handheld belt sander has an earthing strap on theextraction outlet (it's about 3/4" diameter!). It's another reason why you need high air velocity - the scouring effect which will stop a film of fine insulating dust building up on the inside of the piping.
I've read Bil Pense, admittedly ages ago. Some of it doesn't ring true, because of simple physics. And when you visit commercial workshops, you quite often see long runs of smaller piping (4" or 6"), but rarely long runs of bigger pipes, even though the final trunk to the cyclone extractor is often quite wide (but relatively short!). I've seen pipe runs that would be very, very awkward to clean out if they clogged (e.g. 100ft of horizontal run 20ft up near the roof), but they have a stonking great fan at the far end, and crucially, high velocity air moving through, so they don't clog.
If you're intending this for a commercial workshop, for goodness' sake get some professional advice before committing to something that may be quite a nuisance to put right later.
Simple 1st-order calculation of relative air velocities for pipe diameters, compared to a 2" pipe:
2": 1
2.5": 0.64
3": 0.4444
4": 0.25
6": 0.1111 (i.e. only slightly more than 10% of the 2" pipe)
This assumes the pipe is frictionless, which is far from the case. Turbulence in the smaller pipes will mean the numbers are slightly better for the bigger piping, but not by a lot. You get the general idea: the velocity drops by the square of the relative difference in diameter.
I note in passing that this is the main reason why dust collecting hoods for things like mitre saws are relatively useless. At the outer end of the hood or chute, the air is moving a lot slower than at the "nozzle end". But the sawdust is moving fast and has the kinetic energy to easily escape the pull of the air gently moving towards the extraction hose. It works on a tablesaw to have a chute at the bottom, because gravity aids you - the dust has to end up on some horizontal surface somewhere, and letting it slide into the suction pipe works. But it won't get the dust to shift off the mechanics and crevices of the saw for the same reason - the air simply isn't moving fast enough over those parts of the saw.
Hope that makes it a bit clearer.
E.
PS: the springy metal spiral often found in plastic concertina piping won't help with static if it's moulded inside the plastic! I have some 4" tubing like that, BUT the plastic itself is conductive. It also helps if that metal spiral is exposed on the inside surface of the pipe. The idea isn't to connect n earth between fittings along the pipe, but to discharge the static build up on the surfaces inside - that's a lot harder if those surfaces are not conductive in the first place
