This is up to you and your dentist. It is better to be a little undersized than oversized to prevent gag reflexes. They fitment is comparable to a sports mouth-guards. 80% of the adult population should fit comfortably into the regular EZ Teethbrush mouthpiece size. Some smaller sized mouths may prefer the adult small. There is also chart on the campaign page for child age group recommendations.
Yes, you still have to brush. Simply use the Dentist Recommend "Modified Bass Technique" which uses small circles with light back and forth motions to get the inside of the teeth and gums and the outer surfaces.