Here is a extensive article on this subject:

http://www.woodcentral.com/articles/...ductwork.shtml

tr wrote:

I have a 2 hp Jet dust collector...my first...there are two 4" intakes
on the unit....is it best to run 4" hose around the small shop rather
than 3"...etc...and when dropping to a tool...should you keep using
mostly 4"...or is it ok to drop down to 3" or 2 1/2" at the split off
fitting to the stationary tool ?

Thanks, TR