Djnieve, I think my anus has indeed stretched with practise. I can now get about 3 1/4" up me provided I go slowly; but when I began about five years ago, could hardly manage even 2". I began with those glass spice jars the supermarkets sell. They're only a bit over an inch, and you can get one in and hold it there a while. Repeated insertions are a good idea, too.
I gradually progressed to cucumbers, small apples, larger apples, potatoes (don't like them much - skin too rough) and butternut squash. Veggies are safer than glass!
But the most important part is relaxation and bearing down as if to take a shit. So start off clean to avoid unpleasantness! Then go s-l-o-w-l-y. It will hurt, but it should be a 'hurts-so-good' kind of thing, and not torture!
I discovered something else that may help. Bulb syringes are available in a range of sizes usually, from quite tiny childrens' ones to large toys for kinks like us! Well, find one whose diameter is a little bigger than the anus can now take. Lubricate well (anus and bulb). Relax anus after inserting biggest object that will now go in. Squeeze bulb until it collapses oompletely, then cover the nozzle hole with a fingertip. The collapsed bulb will now stay collapsed, and in that state (like a flaccid retention nozzle), you can work it into the waiting hole until it is fully inside the sphincter, with only the tip protruding. Remove finger from nozzle hole. Now ask the owner of the anus to suck his/her abdomen inwards, to relieve pressure on the outside of the bulb syringe buried in the rectum. It should inflate with a sensible thud. Now the finger is placed back on the nozzle, and the person holding the bulb is invited to bear down as hard as possible to expel it.
It is much easier to expel large objects than it is to insert them (experience speaking, here) and the bulb should come out without too much trouble. As it does so, it will stretch the anus delightfully; and the finger on the nozzle prevents it from collapsing again as it emerges.
By the way, don't worry about a stretched anus not holding shut properly. I've never had a problem.
Johnny