use at lest 8" vent smaller will make the fan ...
919176
use at lest  8" vent  smaller will make the fan  little bit loud , but looks sharp
799068
I had 16" versions of this fan at our summer camp.  I needed one for a smaller room so I bought this 12" version.  It works great too!
4000019
Works very well with Fantek fan.  The air restriction is minimal, making the most exhaust from my $300 inline ducted fan.  These vent covers are not 100% air tight when fan stops, allows some cool fresh air to back draft.  Recommend using an in ceiling fan with an air flap if cold draft bothers you.
234627
Hard to rate - this filter is about 2 inches shorter than the original filter (didn't see other sizes, might've missed that) so it doesn't entirely wick water upward into the air column created by the fan. Evaporation is thus about 1/3 slower than normal.  Given that we have the 1 gallon humidifier, I can't imagine a smaller size exists, but  - we'll try to find the taller filter.
3377340
It doesn't fit great in my vent, and I worry about using the heat from the vent, but I do like how I can see the screen easily.
2353249
This fan and filter works well in my 4 by 8 and it is rated for larger tents such as 8x8. The fan noise is noticeable but I drowned it out watching TV so it's not too noisy.
1047402
Fan is good but it does not put out as much air as I would have liked. Wish it was a little bigger.
914599
I have a smaller vent in a bathroom and it sqeezed in nicely. Was worried for a minute that it wouldn't work but my wife called and they assured one size will fit both.
305413
The measurement is for only the vent part, not for the vent and the outside piece around the vent. Otherwise it's nice.
439470
Smaller than I expected, looks nice. Adds a little more noise level and air flow which was the goal with this muffler.
821070
It works great! I am using it in conjunction with a 6" TerraBloom duct fan.

I use it in a small tent that measures 18"X18"X48". I set the fan vertically on top of the tent to extract warm air. It works perfect - I was debating between a 4" fan or 6" and I decided to use the 6" with the speed controller to run the fan at a slower speed in hopes it would be quieter than a 4" fan running at full speed, but still move a comparable amount of air. In other words if I used the 4" fan it would have to run faster and make more noise to move the same amount of air. Something to consider.
4374716
What this fan is designed for, is to fit into very small spaces, like between 2x4 stud dry wall spaces then to run out a flat or 3 inch duct to an outlet.
Good luck finding an adapter -no, this doesn't come with one - as the fitting on this fan is a 4" OVAL.  I had a 4" to 3" steel adapter handy and just crushed it in a vise, cut the flange and used pop rivets.  This is going into the ceiling of a basement bath to replace a LOUD 50 cfm fan that is going bad and will go out a 3 foot piece of 3" ducting.  I've plugged it in and it's VERY VERY quiet, nearly whisper-quiet, which should work just fine.
4087078
We have 8" fans and it won't push the flap open, no one want's to get back to me as to what I should do next.  Never had problems with the Broan brand.  Lesson learned.
1298221
Do your homework on this one. It will only fit (just barely) in one of my ceiling fans. They take time to come on full white as well.
4588102
Less range of motion than desired as the holder does not protrude far enough from the vent to angle it fully as desired.
555947
Doesn't fit an 8 inch pot as the base is much smaller than the edges that fan out at an angle.
3379218
the idea was a good one, except my air conditioner was shorter in depth than the bracket, and I could not make it fit. I will keep it just in case I end up with a larger unit.  I will keep looking to find a bracket that is a bit shorter in the mean time..