The process of anal stretching is a lot like earlobe stretching, except you don't wear the "jewelry" around the clock. Like stretching a piercing, it involves slow and increasingly larger play over an extended period of time. Like all good things, it takes time!
Done carefully, anal stretching can safely (and without negative side effects) result in rectal circumferences of 15" (38cm) = diameter of 4.77" (12cm) and more. Done slowly, the process should be pleasurable (and may be a little painful as well). If there is bleeding or excessive pain, you're stretching too fast. Always be sure to use lots of lube.
After stretching, it takes three to six hours for the anus to return to normal (it will have large swollen "lips" until then). Other than the ability to engage in heavier anal play, anal stretching will cause no functional change to digestion or defecation.