Answer
There is no critical mass for fusion.
Work Step by Step
Too much fissionable material in one place (above a "critical mass") can undergo a spontaneous chain reaction starting from just one ejected neutron.
On the other hand, fusion fuel, much like chemical combustion, requires a high temperature to start the reaction. It is like combustible fuel, e.g., gasoline. It has no “critical mass,” and won't spontaneously ignite.
