don't no what ducting fan I should get for exhaust?

I have a 400 watt mh/hps cool tube temps are 77 to 79 with 4in 120 cfm inline booster fan exhausting air out. My box is 3Wx2.5Dx6T I was thinking I would use the booster for intake and get a squirrel cage fan 6in 400cfm for pulling air through the carbon filter through the cool tube and exhausted out. But I'm not sure would just like alittle bit of help. Should I go with a 4in or 6in my cooltube has a 8in port on it but 8in wouldnt fit in the box.

#newbie

you would be good in that space with either a 4" or 6"....I had a similar space and light used 180cfm 4" vortex inline and it worked fine. I always wish I had a 6" inline fan with more CFM and speed controller to dial it in tho. Also if you ever outgrow that light then the 6" can be used with larger systems...

I have a very similar space, 2x3x7 closet with a 400w digi ballast. I have passive intake and run a 6" 454cfm can fan through an air cooled euro reflector. Temps are as low as 71ºF when running on high, def get the speed controller. It is cold here right now so temps not an issue. I run this thru a 4" carbon filter, I will be getting a 6" carbon filter to take the added stress of duct reduction off my fan. But it works great for now. Here's a link to my grow, on this site.
https://www.rollitup.org/newbie-central/779287-2nd-scrog-400w-delicious-cotton.html#post10091050

Btw I went 6" over 4" because the cfm went from 180 to 454cfm ! And then power usage was around 80w on the 4" to 120w on the 6", so it is much more efficient.

I concur with rwbrock. Can't go wrong with a bigger fan, you can always grow into it. Also get a fan speed controller and you can tune it in!

Variac baby! Note some other speed controls make your motor groan at reduced speeds.