Oral, lube, patience...... Try starting off with a smaller dildo or doing the old "One finger, Two fingers, Three and more.." Once the initial discomfort is over and she/he is into it, I don´t think it is a problem to keep stretching the ass. (Up to a certain point I guess). My girth is 17cm circumference and I have had about 30 partners where anal sex has been mutually enjoyed. No problem... Good luck.