I would go with the 6" because you don't know the heat characteristics of your cab. You may have the 5 installed and your temps may be to high and your will wish you had the 6 or even an 8.

Matching the duct size of the cool tube and the blower is not as important as matching the the cooling requirements of your cool tube to the blower. Your can always get duct reducer that goes from 5 to 6 or virtually any size you want to any size you want. I run 8 inch ducting and my hood duct is 10 inches.

Filter - cool tube - blower - Vent to outside your home.
If it vents inside your room, your room will get very warm.

I don't use a filter I just have my duct vent to the outside up high. Its about 30 feet off the ground and on the down wind side of my building. I can't smell anything anywhere around my building, because the vent location blows it to heaven.

Your should also try to remove that wiring from the back wall of your cab. Its hard to tell if its electrical or phone. Either way back there it has a chance of getting wet and burning your house down.