To ensure a comfortable and secure fit, Conveen Optima comes in four different sizes: a standard length (25, 30, 35 and 40 mm diameter) A standard length Conveen Optima male external catheter will generally be suitable for a non-retracted penis. If the size of the penis is found to be in between two sizes, choose the smaller size. Use the Conveen Optima measuring guide to select the correct size of urisheath for each new fitting.