What size duct?

I have an 8" 750 cfm. I wanted to know if I needed 8" duct or if I need to go 9" and clamp or tape it? Also I will be venting to attic so I wanted to know when I put the hole going to the attic if I can make it a 4" and just get a reducer say 8" to 4" or will that back things up trying to push through a smaller hole? Will it cause any problems?

8 inch duct will attach seamlessly with your fan. If your going to reduce from 8 to 4 inch id also get a fan speed controller. But preferably you make a 6 inch whole than reduce 8 to 6 which isnt to drastic. Especially if you have filter attached.

You would still need clamps all around.tape as well if you want.

okay cool. I have the speed controller all ready also. I will try the 8" to 6" if that does not work well I can just make the hole bigger later. Thanks

8 to 4 would work if you have a speed controller low enough. But id stick with 6 you wont have any problems.