Sizing Of Ventilation Duct

I have 21 feet of duct to install to vent my basement bathroom to the outside. This includes 4 elbows and maybe a reducer depending on whether I use 6 inch or 4 inch duct. The fan specifications are:
1/20 hp 1.95 amps 270 cfm at free air and 250 cfm at 0.05 static presure and on down to 70 cfm at 0.375 static pressure.
The bathroom has 556 cubic feet area. The blower and fan are mounted into a steel box that has a discharge flange opening of 3.25 inches by 10 inches. I think this indicates I should use 6 inch duct but I need confirmation. But if I go with 6 inch, I will need to reduce it to 4 inch to accomodate the 4 inch opening in the foundation wall.
So which size should I install? Is 19 feet of 6 inch duct reduced to 4 inch at the last two feet better then installing 4 inch for the whole run?