The frame size of a Smith and Wesson model 66 is K.
K frame
I believe it to be the K frame
N
The M19 is a medium size K-frame. The M28 is a large size N-frame, which was also used in producing the 41 and 44 magnums.
By the size and features
Need to know barrel length and ALL markings.
S & W MOD 65 IS A STAINLESS K FRAME SAME AS MOD 10 AND 19 ETC
Smith & Wesson refered to this model as one of their "Kit Guns" meaning you could carry in something the size of a fishing tackle box. The M#650 was stainless steel, small frame (their "J" frame), with a round butt and a 3' barrel.
Compare it to known examples.
30
Size.