Can you explain this one? I don't think it follows. A given transaction should have common sizes, yes (see: CoinJoin), but I don't think it follows that all transactions need to have that same transaction size. Each join can use a facilitator-chosen size so long as the wallet is capable of tracking the anonymity-set implications for each of its outputs.
Sticking to a few common coin sizes should improve anonymity in fewer rounds, even when using CoinJoin, and even if your wallet helps keeps track. The simple reason is that if your coins are already in typical denominations, you can participate in another mixing round immediately without having to make change. On the other hand if every mixing round uses a different size, you will always need to make change, and then would need to mix the change as well.