It HURTS (eee-YOW!) without a flush. My big ones were over a centimeter in size (I didn't measure them precisely, however). Actually, they were a centimeter or more across on each face, and the stones were kinda like many-sided (i.e. role-playing game style) dice.
I'm assuming they were calcified -- they scratched my sphincter (what the heck was that?!?) on the way out. They were hard, but I didn't have them analyzed for calcium content.
Many others of us have, too, and you can see many pictures of the things which have come out in the image section on this web site.
BTW -- passing the large stones during a flush did not result in pain (unless I slept through it...).