Bill is, of course, correct. The movie guns were Model 29-2s with 6.5 inch barrels.
An 8 3/8 inch was used only for the movie poster for one of the movies, The Dead Pool, if memory serves. The 8 3/8 inch revolver did not appear in the film. Nor did Model 25s, 57s, 19s or any of the other models frequently mentioned.
Archie Dubia, a long time S&W employee is credited with actually building the 29-2 used in Dirty Harry in S&W's tool room as the model was not in production at the time they needed it for the film (no, it was not temporarily discontinued - they just don't make all models at all times).
In an effort to explain the short supply of Model 29s at the time, Skeeter Skelton, I think, in an article about the Model 29 which appeared in Shooting Times in the early to mid-70s said that one reason the 29s were in short supply (I waited a year for mine at the time), was that N Frames were used for other popular S&W Models at the time including the Models 27, 28, 57, 58 and even the Models 209, 210, 276 and 277 (not a misprint).
And, for clarification, "S" is a serial number range for post-war N frames, not a separate frame size.