I have a ton of plugs and have been using all sort of items for anal gratification for the last 20 years. Currently by favorites are a large Sasha Grey clear glass plug and a small (1.6 dia.) stainless steel plug called "The Graduate". I have basically worn plugs daily for the last several years. Granted, I may go for a short while where I go without, but you can always count on me to either have a plug in the ass, a heavy ball stretcher on the balls and a stainless sleeve on the cock, sometimes all at the same time, sometimes one at a time. I am also a firm believer in regular ememas to keep me cleaned out. Additionally, I have been wearing thongs (made for men) for over 5 years now.
The largest I have ever tried to "wear" was the "Hitch" by Tantus. This thing is not super huge (about 3" dia.), but it has virtually no taper (think 3" silicone ball, with a very slight taper to the flange (again 3"). Usually, after some play and relaxation it goes in easy enough, but if not continued to be played with and left in over time, taking it out is a real pain in the ass! I have also used "The Fist" and "The Hand", with limited success.