Tbh I wouldn't be surprised if the sizes are not quantised at all. But if you look at the histogram in [1], most synapses fall into the low number of states range. This is probably related to the aforementioned sparsity of certain neural network layers. Pruning outliers from the brain is really difficult from an evolutionary perspective, but the approach linked in this post where you simply treat them differently than other parts seems like a reasonable way to go for artificial neural networks.