When I first built my fat bike, it was an experiment to try one out. I had collected a few bits and picked up a pair of relatively cheap wheel to suit. They happened to have 100mm rims. I ended up fitting 4.0 tyres as that’s what I had.
Although it was great fun, it always had slightly peculiar handling – tram lined like a bugger and was super sensitive to camber on the road and trail. I experimented with a 80mm front (as I picked one up). It seemed a bit better but the back didn’t want lean over. Finally splashed out on some Halo Tundra 80mm rims and fitted to the back to match the width of the front. What a revelation. Handles properly, turns well and dropped about 150 grams. All round an interesting learning process.
It also proved that you could just about fit a 190mm axle in a park tools wheel jig which was a relief!