Moderate. Basically anything that can get down the ureter into the bladder, can easily pass through the penis tubes (urethra). If your stones are in the kidney, they may be in the pelvis of the kidney and can be quite large. A piece of this could break off and cause the famous, painfull kidney stone passage through the ureters. You should be evaluated by a urologist for kidney stones before this happens.
Stones. 3 mm or less have greater than 75% chance of passage, above 5 mm drops below 50%....Penis isn't the problem usually, problem is getting to bladder from kidney...Pain from blocked kidney.