It used to be that
CIC hearing aids were the smallest option available but now you can go one better for the ultimate discretion with Invisible In Canal (IIC) hearing aids!
These marvels of miniaturisation sit even further in the ear canal so are virtually invisible. Their faceplates (the upper section where the microphones and battery door are) are usually made in a darker colour too so are further disguised. In fact the only "visible" part is the small, clear plastic removal handle. IIC hearing aids are custom made (where we take ear impressions which the manufacturers use to build the instruments) for the perfect fit and are more suited for a mild to moderate hearing loss.
Like other fitting types, size will vary between brands and you may also see them called other names like Widex' CIC Micro.