My father was an FBI agent from 64-84....
In New Agents school, he was issued a standard barrel Model 10. He carried it in a "Hank Sloan" holster. He bought a 2 inch Model 10 that he carried throughout his career. I have that personally owned Model 10 now, and I still carry it at times. Somewhere around 1980 Dad was issued a 3 inch Model 13 and he surrendered his original Model 10 that he was issued in New Agents...
According to my father:
He liked the 4 inch M 10 fine, it was just awkward to carry under a suit.
He liked his 2 inch better....and he still asks me about it all the time.....
He LOVED the 13, but who doesn't....
He doesn't recall being issued a 2.5 inch, or other Model 10....