Kidney stone of 12mm size?

Yeah, that's big. A stone up to 5mm in size will predictably pass, with variable degrees of pain. A 12mm stone is going to get stuck if it decides to move out of the kidney. You should get that stone managed before it starts to move.
See Urology. If symptomatic need to se urology ; may need intervention depending upon the location. If recurrent to see a nephrologist for special testing and treatment. Meanwhile increase water intake if ok to do based on other medical or health problems and low sodium diet.

Related Questions

Can kidney stone of 8mm size cause rise in bp.

Yes If obstructing . If its an obstructing renal stone causing hydronephrosis, urinary collecting system dilatation, you are most likely also have associated symptoms of flank pain possibly radiating to the groin, +/- fever and blood tinged urine.The stone and the pain associated with it can cause your pressure to rise. BP will fall back to baseline when the stone passes and the pain resolves.No if it's not obstructing. Read more...

Have got kidney stone measuring 0.5mm. Is it big size as the pain keeps coming and going on my side. Is the size big to worry of?

Small ones are more. likely to induce pain. Why? Because stone surfaces are composed of crystals with sharp surgaces plus ureters are typcally both very sensitive to irritation & constrict in response to pain, thus visious cycle. Big stones in renal pelvis do not readily enter ureters. Small stones more easily pass into ureters. Always catch stone to identify which of 4 types with goal to help prevent recurrances. Read more...