The GF fan is around 200CFM we are told, so I think it is best to be close to that.
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I am using the fan (without the filter) from this:
https://www.amazon.com/dp/B00D7M6692/ref=cm_sw_r_cp_tai_-2vOAbP8KFDYS
I run the stock 8’ 
hose into this fan and then from it an additional 20’ hose out through the sliding glass door and into the back yard. Seems to be working without any issues.
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I took a look at what I have and it seems the smallest I have is 863CFM. :-o
Many people have tried. It’s all over the forum. One of the things to watch out for is that most fans are not sealed and smoke will leak from every seam.
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I got one off of Amazon - Diameter: 4 inch; Air Flow: 100 CFM; Speed: 2930 RPM; Voltage: 110/120V; Wattage: 12W; Noise Level: 40 db. But I’m rethinking I need a higher CFM based on @palmercr recommendation. I’m going to do a little more research.
https://www.amazon.com/gp/product/B00CQBFOTS/ref=oh_aui_detailpage_o04_s00?ie=UTF8&th=1
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I’m using this booster fan: https://www.amazon.com/gp/product/B00AL0DJNY/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1
It’s 200CFM, because I was trying to match what the GF had. I used 4" sewage pipe for my 10’ run. I do have 4 turns (2 90° and 2 45° elbows) .
When you open the lid, you can feel the booster fan air sucking inside the GF.
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I’m using this same fan now.
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Did you ever order this particular fan and install it? I’m curious if just a little booster will work well enough for my needs and this is well within budget.
I’ve just been using the built-in exhaust and run the vent straight out a window through the length of the tube. It’s been a little shy of okay - only a problem when I cut acrylic and the fumes linger. I don’t mind the smell of wood smoke as much.
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Yes, this is the one I have in my photos. I have it plugged into a surge protector along with the 
and a air purifier from Home Depot. The fan and the purifier come on when I turn on my . It works great. I set it up because of the problems with cutting acrylic and the fact that I have it in the basement off of what we use for a family room. The little boost from the fan really helps with smells and smoke.
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Perfect. I also run an air purifier when cutting and acrylic is what bothers my husband the most.
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For 12 feet, I would install a booster rated around 190 CFM. Best configuration would be to mount the fan as near the end as possible, like right at the window. Any of the run that is downstream of the fan will be pressurized - so every seam will be prone to leaking, while any of the run upstream of the fan will be under negative pressure. You want any potential leak sucking air, not blowing.
Since the booster fan runs even when the machine fans are off, it helps keep the stink down. Speaking of stink, CA glue bothers my olfactory, so when I glue something up - I put it inside the glowforge so the fume is evacuated. 
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Hooray for multitaskers. Glowforge as a fume hood!
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I use a 100 cfm inline booster fan ($20 on amazon) with two 90 degree-ish bends with rigid vent pipe. Same as others here, it gets plugged in and run the entire time I’m projecting. There is always gonna be some smell, since the cut items themselves stink! Although, it isn’t super sucky, it is sucky enough, since I only really get a smell when my is opened. May consider more cfm, but the setup I currently have works great.
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I use this one, and it’s quite effective:
https://www.amazon.com/gp/product/B01M7S46YZ
I picked up 25’ of this hose at the same time, and it’s much more durable than the one provided.
https://www.amazon.com/gp/product/B075JB7VG2
Finally, I also picked up one of these filters. It doesn’t eliminate the smell completely and I still vent outdoors most of the time, but if I need a quick cut on most wood it works great.
https://www.amazon.com/gp/product/B01CJ5D4AG
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Re: fans and filter setups then. I’ve learned quite a bit in the research I did for building mine for the forge.
Filter-wise, I went to this post and built roughly the same thing. I also included a large G4 bag filter in there too and it does a good job of screening all the smoke out so far:
The completed unit is big, and awkward and heavy and I really should have paid more attention to the space i had to work with. I’ve also no idea how it does with the really small, dangerous, particles lasers produce, but I vent the exhaust out the front of the garage via an under floor ducting.
Fan-wise I read this really useful Reddit post on bouncy castle blowers (bounce house, for the Americans) being great for this purpose as they are good at driving large volumes of air against static pressure quickly. They’re also a lot quieter and more efficient than shop vacs or inline fans for kitchen / bathroom vents …and second hand, they’re cheaper too! just, don’t use them as a booster fan with no back pressure as they just run faster and faster until they blow up without any load on them!:
https://www.reddit.com/r/lasercutting/comments/5f0wmn/on_extraction_fans/
The one thing I definitely am stuff struggling with is the smokey smell. I have incorporated a pleated carbon trap in the filter system but that’s really not cutting it. I’m going to install on of the hydroponics filters below on the output vent, but these are design to filter air being drawn in, not vented out so I don’t know how good it’ll be. I’ll report back:
The one I used came with instructions showing it being used in either direction, and reduces the smoke a little, but doesn’t really get rid of any smell. I would never cut acrylic without venting outdoors.
I run my line to a small enclosure outside my window with a car air filter in-front of a bathroom exhaust fan on a dimmer switch to control the speed.
But I tend to build with what I got or can find rather than buying new, so can’t tell you what CFM it is at full speed but I haven’t had any issues.
Basically it goes GF–> GF vent hose–Air filter->Fan–> nature.
Okay… I wonder how many of us are now on some list from all the “hydroponics” equipment we are purchasing??? 
BTW, I found that 200 cfm was the sweet spot for me with a fan speed control. High for stinky (all) cuts, and low for afterwards and general room evacuations.
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Yeah, I have a bunch of equipment for an aeroponics system I want to set up for growing herbs and vegetables year-round. When I asked on another forum if anyone had experience, I was mocked and asked exactly which “herb” I planned to grow! lol… I had no idea THAT was as popular of a product to produce yourself…
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DEFINITELY on the “list”. They’re watching you!!!