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

The masseter is used to close the mouth and clench the jaw, opening would have no effect on the botox or the masseter size.  Best, Dr. Emer.

Keeping your mouth open will have no effect on your masseter muscles.  You can try to avoid activities like chewing gum to help maximize your results. 

The answer is absolutely not. You have four primary muscles that move your jaw. Jaw Elevators (Close your Mouth) - The masseter, temporalis and medial pterygoidJaw Depressors (Open Your mouth) - Lateral PterygoidOpening your mouth does not affect the masseter.Dr James P. Bonaparte, MD, MSc, FRCSCOtolaryngology - Head and Neck SurgeryFacial Plastic and Reconstructive SurgeryOttawa, Ontario, Canada

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.