what size fat tire bike should I get?
 

The numbers associated with frame sizes don't really have any relevance.. every manufacturer measures their frame differently and there are other geometry considerations beyond a single number that (probably) corresponds to the length of the seattube measured from the bottom bracket to …some other point. I think using a S M L XL sizing system is more useful as some companies call their 20" an XL and some call it an L, likely based on the other details of the frame, and who they intend the bike to fit. SO if you are asking 'should I get a 17, 18 or 19" frame', the best answer is 'Yes.'

For a 5'11 person I would say a M or L size will likely work. Make sure you have standover clearance (so you don't mash your crotchular region when dismounting), and can raise the seatpost enough to get proper leg extension without getting too close to or past the min-insertion line on the seatpost. THe seat height and fore-aft position are adjustable, and you can further fine-tune the fit by changing stem angle and length.

Generally speaking, if you plan to do aggressive off road riding then opt for a smaller frame, and if planning on less aggressive off-roading you can use a bigger frame. Smaller frame will allow you to maneuver the bike around, and maneuver yourself around the bike, better

I’m 5’ 7” and ride a small Scott Big Jon, a friend of mine is 5’ 8” and rides the same bike and size. I feel you should look at a medium size frame however, every person is different so test ride first if possible. I do feel stand over height will play a big part in your decision.

Start out with what manufacturer recommends. You probably will be an L. Don't get a too small or large bike! Modern bikes have more reach to make up for shorter stems.

Standover isn't used for sizing, but pick a frame that has a low-standover design. they have a steeper downtube (going steeper towards seattube) and some sort of gussett to make up for structure. Look at Mayor RSD for what i mean. Cheaper bikes just have a straight slanted top tube, they will have a higher standover at a given frame size.
If you so any technical or snow riding, you want LOW standover. Trust me, i upgraded from a high to a low standover frame and I thank myself every time I get stuck over technical terrain. You know, your foot often lands below the tires.

I have only ridden two fatbikes. One was a Specialized Fatboy (which I think is now discontinued) in size large, and the other is my ICAN which they call a 20" frame which is their largest size. My experience with the two tells me that overall geometry and weight are more important than pure frame size. The ICAN has a slacker head angle than the Spec, which makes it feel much more stable on faster down hill trails, and it is lighter which helps it respond quicker.

For the kind of riding you are describing, I doubt that you will notice much difference between a "Medium" and a "Large", or a 25 pound carbon bike or a 35 pound aluminum bike.

I think you just need to take a test drive and find which one you like.

I'm 5'8" and ride a Surly Troll Medium with 26 x 3" Knard tires with 2 x 11 drivetrain. The thought process on building this bike was akin to "Compact Utility Bike," so get groceries for 4, pull kid trailer, ride on the beach, Chicago Snow/Slush; hopefully some bikepacking this summer.
I'd also test ridden a Medium Surly Wednesday and Kona Wo. Both great fun but a lot of rubber to push around for day to day riding.
For longer distances, after about a year or so, I got a second wheel set so I can swap out some more city friendly wheels with Schwalbe Big Ben 26 x 2.125.

Best of luck.

At 5-10 you fall in that zone where you may end up on a med or large, depending on the bike model, your proportions, and preferences.

This is exactly where I was at when I bought mine earlier this year. I ended up on a 'Large' Surly Ice Cream Truck, but due to the bike shortage, this was the only size available and I couldn't try out a Medium. I've been really happy with the fit and have had no problems maneuvering in the snow or our river bottom single track trails.

I'm 5'10" and a 17.5" Motobecane Sturgis fits me, though it's a "touch" larger than I'd prefer. They consider it a medium.

i ended up getting the medium, 18 inch, feels just right, sitting is not too upright, not too leaned forward, it's great, gonna do a review on it soon

The critical dimension is the distance from the saddle or seat post to the front handlebar. This affects your riding posture and how far forward you need to reach to access the shift levers and the brakes. This dimension varies from one bike to the next regardless of the "size" of the bike. I would go to the nearest bike shop and get on some bikes and see which type and size are the most comfortable. The amount of front fork rake also varies and can make a "pro" or "expert" level bike seem overly sensitive and squirrelly for a novice and this will be noticed in particular on downhill runs.

Second key dimension is the standover height so you can stop and not have to worry about damage to the crotch area.