 In order to connect the probability "density" in phase space to a probability "distribution" over microstates, it is necessary to somehow partition the phase space into blocks that are distributed representing the different states of the system in a fair way.It turns out that the correct way to do this simply results in equal-sized blocks of canonical phase space, and so a microstate in classical mechanics is an extended region in the phase space of canonical coordinates that has a particular volume.In particular, the probability density function in phase space, , is related to the probability distribution over microstates, by a factor
where
Since can be chosen arbitrarily, the notional size of a microstate is also arbitrary.