Experts and BHG readers answer.
Mums in small planter boxes
If you purchase small mums already budded up, they won't get much larger this year. The only additional growth that they'll put on is the extension of the flowers. The overall size of the plant should remain essentially the same.
Other varieties of mums can be kept quite small may pinching them back every couple of weeks until mid-July, at which time they'll begin to set flower buds.