I had Botox on my masseters. Does keeping my mouth open for 30 mins. or so (dental cleaning) make my masseters big again?

Your masseters will not be affected by opening your mouth widely for dental cleaning. Remember that Botox only lasts about six months.

     No, the masseter muscles are activated in closing the mouth such as in chewing, so opening should not affect their size.  Prolonged activation such as in chewing gum can result in masseter hypertrophy so that should be minimized to avoid increasing the size of the muscle.