I made some assumptions because you haven't given all the information and I ain't know engineer. (hammer) but it should get you in the ballpark.
Playing with an online calculator the 110mm (4.375") PVC pipe and 10 meters (33'), a pair of elbows (stamped to be equivalent to PVC) and a guess of 2040CM/H (1200cfm) you have a theoretical static pressure loss of 16" of water column (you'll have to convert that) and velocity of 11,500 feet/minute in the duct. That is actually impossible to achieve as the maximum flow a pipe that size can handle would be 450 cfm (760CM/H) so the speed would be lots lower.
Upping to 6" (150mm) galvanized metal (5piece elbows) and there is a loss of 4.3" of WC and the duct speed would be 6100ft/min. So 6" would be the minimum you want to get the flow if my guess is correct for your DC. Advertised flow rates are exaggerated by as much as twice of what can be achieved in an actual system.
Going to 7" (180mm) galvanized metal (5piece elbows) and there is a loss of 2" of WC and duct airspeed would be 4,500ft/min. The speed in a duct you want to carry dust is 4,000ft/min as a minimum so this would be the maximum you want your duct diameter to be.
The suggested 8" would be a loss of about 1.1" of WC and the airspeed would be 3,439ft/min. This would start being too big and the heavier dust will settle out in the ducts especially if they are stepped down along the way. I calculated it as 8" all the way as the calculator doesn't let you do multiple sizes. You have to do them separately and add them up. I think. :roll:
So you would only want 7" pipes if your runs are basically straight. 6" will give you the flow you need as long as you are not stepping the pipes down. In other words you need to cut the hoods, chutes and ports open to 6" to allow the air to flow. You can use smaller ducting and openings but you will only move the chips and not the fine dust that harms the respiratory system. Basically what you now have.
The calculator I played with if you want to put more accurate info into it is.
http://www.freecalc.com/ductloss.htm
Bill Pentz did a lot of research on dust when it made him very sick and among other things his site has a spreadsheet for designing ducting. I suggest you plod through the whole site, even though it is repetitive, and you will understand it all better.
http://billpentz.com/woodworking/cyclone/index.cfm
Pete