Is the size correct in specifications? I have a fluidmaster 5103 sitting in front of me, and the diameter is 3". But specs say 4" here.

Yes, the size is correct. Flappers are strange in the way they are measured. If you measured the diameter of a 2" Flapper, it would most likely be somewhere between 2 1/2" - 3". Where as a 3 inch flapper should measure out to somewhere between 3 1/2" and 4"