It is essential that the toothbrush can move freely in your mouth to clean every tooth surface. The bristles should be soft so that it does not injure the gums.
The size of the toothbrush head should be appropriate to the size of the oral cavity. In general, the size of a toothbrush head which is the same as the diameter of a one-dollar coin will be suitable for adults.
If you choose to use an interdental brush or a powered toothbrush, you should consult your dentist first.