I use carrots and you can choose a size you like, I trap them in a drawer and back on to them, the feeling of the small end entering and getting bigger as you push up to the thick end is magical
I've used wife's dildo and various butt plugs. I've also tried different kinds of produce - small plums, a lime, roma tomatoes, carrot, zucchini, cucumber. Cucumber is probably my favorite. I can get smooth, bumpy, curved, medium, large, or all the above. Roma tomatoes are also great for keeping inside for a while. They hit just the right spot inside me!
When I was 13, I found out the legs of my dresser could be removed. It was probably a good 8 inches long, and fairly thick. I put a rubber on it, and and found out real fast how great it felt to have something up my ass.
A cucumber would fit I'd say. Nobody would be any the wiser. Best not to insert it too far or the emergency room at the hospital will have a laugh trying to get it out of your ass.