It depends how large you plan to grow your moms. I keep foot tall moms in QUART pots but I regenerate them from clones frequently, and 'mow' them and remove branches when they get too tall. There is a thread on keeping bonsai mums somewhere that is a good read. I have not root-pruned mine yet but that would be part of doing it correctly. If you need a nice big mom with more than 6 tops, go for the 3 gallon pot.