The kidneys remove wastes and excess fluids from the blood. The waste products include minerals that are normally dissolved in the urine, but they sometimes separate out from the urine and form into tiny crystals. Over time they clump together and harden into a kidney stone.
Stones can form if there isn't enough water in the urine to completely dissolve waste products or if there is a high level of the substances that typically form into stones---calcium and oxalate. Urine normally contains chemicals that help stop crystals from forming and if these are low then there is an increased chance of a stone forming. Some medical conditions such as urinary tract infections and kidney disorders are linked to a higher chance of stones, and chronic inflammation of the bowel can cause calcium oxalate stones.
Most sources state that the size of a kidney stone ranges from being as small as a grain of sand to as large as a golf ball. Researchers at the University of Missouri published a study about treatment for "large" kidney stones in the Journal of Endourology in September 2009. They considered any kidney stone over 2.5cm (0.98 inches) to be large, and the average size of the large stones in their study was 3cm (1.18 inches).
Kidney stones don't need to pass out of the body as long as they remain in the kidney and cause no problems due to size or amount. But they cause severe pain if they move into the urinary tract. This is where size and shape become important. A smooth, small stone passes through the ureter more easily than a larger, jagged one. Any stone will cause pain but if it is too large to easily pass through the narrow tubes of the ureter, pain continues as the smooth muscles try to push the stone through the ureter. Stones that are too large or jagged are more likely to get stuck; then the flow of urine can be blocked, pain increases, and an infection can start.
Physicians at the Delaware Urological Associates state that kidney stones that are larger than 1cm (about 1/2") will "rarely pass through the urinary system without complications." They also note, "For stones less than 3 mm (0.12 inches) in width, the chance of spontaneous stone passage is very high. Stones more than 8 mm (0.31 inches) in width are only about 20 percent likely to pass spontaneously over one year." Stones larger than 8mm may end up blocking the urinary tract and causing bleeding. Stones that are too large to pass are usually broken into smaller pieces using shock waves so that they can pass through the urine. If this doesn't work then surgery is required to remove the stone.