The Beef Improvement Federation recommends a minimum of 32 cm at 12 months of age for most breeds. This is the standard that bulls have to pass for the NY Bull test.
But 36 to 40 is much better at 12 MONTHS OLD.
Limos usually have a hard time passing the 32 cm.
The Simmental bulls on NY test ranged 38 to 41 - mine had a 40.5 cm - all adjusted to 12 months of age.
Black Angus ranged - 38.5 to 44.5
Red Angus ranged - 35.5 to 40.5
Herefords ranged - 36 to 41
Larger yearling testicle size is an indicator that his daughters will be earlier maturity - sexually.
So at 2 years old, I would expect larger than 40. Of course, it all is correlated to the growth of the bull. If he has been stunted, and only weighs 1500# at two years old, his testicle size will also be stunted. (just as an example).