Well a little research seems to indicate the average finger length is only 10 cms so 20 cms could indeed be a challenge. Maybe this is where the idea of the Foam Finger came from.

"Corticosteroids: Patients can also administer these rectally. They come in liquid preparation, thick foam, or suppository, including budesonide (Entocort®), hydrocortisone (Cortenema®, Cortifoam™, Proctofoam®), and bethamethsone, (Betnesol®). However, if the patient has significant diarrhea, then the rectal medications may be difficult to hold. Cortifoam® is a foam preparation of a smaller volume so the patient may retain the treatment in the rectum longer, thereby increasing the amount of time it has to work."