2 mm is nothing I would start flushing right away, that is what I did 6 years ago.
In literature they say up to 1 cm is may be the limit, but there are people here who had larger ones.
Mine is 2+ cm and is probably too big and heavy to come out, but I do flushes so I won't get new ones. There are lot of people who have stones but have no symptoms.
Also read the FAQ at top of this forum and gallbladder surgery forum.