Buying a TD-125 Inline

Newbie question. It says the duct size is 5". Should I be searching for a 5" can filter and 5" ducts. I am having trouble finding a 5" filter.

I am growing two White Widows in a CFL closet setup & a replacement door. I got some people living above me (they are tokers too, though). I plan on lining the ceiling with plywood then a layer of vent filters. Will just one TD-125 cover up the smell? And what can filter do you recommend?

Just get an adapter. They probably sell them at home depot or ace etc. I know they sell them for the normal even sized ducting/fans. If i were you though i would just getthe four or six inch fan and make life easier.