Hi Matthieu,
There is no need for a minimum cluster size because for cluster, the question is "if the null is true, what is the probability of finding a cluster at least as large as this one I found with my statistical test". Putting a minimum cluster size will not change that probability, as the even the clusters that are under the threshold would pile up on the left side of the permutation distribution of the maximum cluster size, as needed for the FWER correction.
And If the cluster found with the test is smaller than the desired threshold, then the permutation test wouldn't even need to be done, such that including a minimum cluster size before doing the test is equivalent to deleting these small clusters at the end, that is, it's an arbitrary decision.
All the best,
Anderson