Put it this way – I’ve run a 2.5/3″ inner tube on my 4.8″ fat tyres for years, never had one go pop! Any size from 1.8″ upwards will work, you could even use a 26″ tube. People worry far too much about inner tube size 😉
When I bought my Krampus it had Surly 3″ inner tubes fitted that weighed about the same as a small star. They also seemed really prone to punctures.
I then started using much lighter Conti 29er tubes (1.8″-2.4″), and not only were they lighter, they also seemed to puncture less.
I went tubeless with it a couple of years ago, but have a Conti 29er tube that I carry just in case. Not had to use it yet!