How many bits should a token have to be unguessable (given some computational resources)?
Well whilst AES can take 256 bit keys, it still only accepts a 128 bit IV. And the width of the AES block is considered secure in 2017. So that would be a good guide.

Also, there are type 4 UUIDs that use 122 bits of uncertainty and are currently in widespread use.  There's not a lot more to say other than to extract this from my reference:-


  Thus, for there to be a one in a billion chance of duplication, 103 trillion version 4 UUIDs must be generated.


Increasing the size to 128 bits would require 8e14 tokens for the same 1 in a billion risk of collision. Should be okay.