When buying a fat bike, there are a few things to consider. Firstly, the tire width. The wider the tire, the more stable it will be. Secondly, it is essential to look at the wheel size. Bigger wheels make for more speed and stability on rough terrain. Finally, it would be best to look at how much clearance there is between your crotch and the top tube of the bike. This is important because if you can't straddle your bike comfortably, it is too small for you!