In order to make things a little easier to organize, I'm just going to go through Carey's frame geometry and make some comments about what we did to make it fat-specific as opposed to his XC bike. Please note that all of this assumes you are going to ride on *snow* or other soft surfaces. If you like to ride your fattie on dry trails, that's great, but designing for that is an entirely different thing.
Without further ado:
-70.5 head tube angle and 44mm of fork offset for an approximate trail number of 86mm (remember that tire sizes and tire pressure can change this significantly when we're talking about fatbikes). ETT is 596mm and front center is 634mm.
Essentially this is just an XC-ish trail number (though a little on the high side). Fatbikes don't tend to achieve super high speeds regularly when ridden on snow so going super slack is only going to cause the front end to wander around at low speeds (which they do regularly achieve) with no corresponding payoff. In many cases you'll need to hold a pretty tight line to avoid falling off the packed section of trail into the unconsolidated snow - so low speed wandering is not good.
-65mm BB drop for BB height of 300mm/12" (assuming 4.5" tires at low pressure).
Unlike a dry-trails bike, you'll rarely strike a pedal on a rock or log, or pedal through a corner at a steep lean angle. So fatbike BBs can be relatively low. But there's also some trail where you're on the packed section in a trench, or riding in deeper/softer snow, and you'll be dipping your feet/hitting your pedals on the higher snow on both sides. Luckily this is mostly going to be softer snow so in general you can pedal through it and leave "footprints" on both sides. If the snow gets really deep you're going nowhere anyway so there's no reason for a super high BB for pedal clearance, in general. Here again, we're basically doing an XC bike type number.
-45cm (actual) and 44.5cm (effective) chainstay length. In this case we probably could have gone a little shorter but there was some debate about what drivetrain and tires to run (we had committed to 170mm rear spacing before 190mm hubs were available) so I was conservative and left them a little on the long side. They could easily come down by 10-20mm if using a 1x drivetrain (which Carey is). Some people really prefer the feeling of a shorter rear end and think that they get better traction when pedaling. Others find a short rear end makes it harder for them to achieve the correct weight balance between wheels. I'm personally a short rear end person.
Keep in mind that the advantages of short chainstays (wheelies and manuals, bunnyhopping logs, etc) are pretty irrelevant when riding on snow in general. In most cases you'll be keeping the wheels on the ground. So this is a matter of personal preference as much as anything. On the fence? I'd recommend some slider or rocker dropouts so you can adjust 20mm and see what you like best (or run as a singlespeed if you're a crazy person!) Shorter than about 43cm chainstays aren't very feasible unless you're willing to let me start doing very weird stuff with the drivetrain.
-Clearance for 4.5" tires. Ah, the tire size question. Some people will say that fitting a Bud/Lou is mandatory to have any fun at all. Others are riding snow on their normal 2" XC tires. Some honesty with yourself will be helpful here- are you going to run around in the woods and have an adventure, not necessarily on a packed down trail? Go as big as you can and live with the restrictions on hubs/chainline/cranks (essentially you'll need to go 190mm or do weird drivetrain stuff with 170) and extra tire weight/cost. If you're going to ride packed trails/groomed stuff or snowpacked roads, you may not want or need the 5" tires and you'll have a lot more options for drivetrain parts and such. In Carey's case we split the difference - the 4.5" Dillingers are pretty close to the chain in the lowest gear and the frame has plenty of clearance for them - but not enough for a Bud/Lou.
-Standard QR dropouts/hubs. With 4.5" of 5psi rubber squirming around it's hard to make the case that through axles are useful for a fatbike, but they are popping up on them anyway, of course. I'm happy to build for whatever axle standard you want but for all around fatbiking there is really nothing wrong with QRs and nothing to gain with a through axle unless you weigh 400 pounds. Then again, it's sometimes a good idea to follow the industry lead if you don't want to get stuck trying to find replacement parts in 10 years. Unfortunately nothing has really settled in for fatbikes yet so when you buy one, you have to accept that your hub/axle choice may be obsolete in a year. Boo.
-Non suspension corrected. There aren't many options other than Lefties right now for fat bikes though I'm sure some stuff will pop up this fall. Personally, I don't care much about suspension when riding on snow as the surface isn't super rough in general but if you're riding on stuff where people have postholed and then it's frozen over or something - might be nice. Carey isn't really tall enough to make a Lefty work well here anyway IMO so it's rigid. Someone needs to have me build them a headshok front end at some point...but they will need to be tall enough to have the bar height work out (probably 6'2" or so and up unless you like your bars really high!)
-A little extra standover from curved toptube. When you put your foot down on dry trails, it usually doesn't sink into the ground and make you land on your toptube/fall over. Usually. On snow it often does so a little extra standover is nice. For folks who want to do crazy expedition riding/racing, of course, this means less room for your frame bag and stuff inside the triangle.