You can ask your dentist or hygienist what size/s they would recommend. Alternatively, you can often buy a variety pack with one of each size so you can experiment to see what size fits most comfortably into the various spaces you have in your mouth. Insert the smallest size first and keep using a bigger size until the brush fits snugly between the teeth without forcing it. This allows for a thorough cleaning action and suppresses the gum from growing up excessively in between your teeth. Excessive growth can cause a natural food and plaque trap which lead to a self degenerating situation.