This cute little thing here - it was between five to seven inches, and was released on 16 June, 1999. After the release of Pikachu, a plush Meowth, Gengar and Psyduck was released the following year in 16 February, 2000 again, between five to seven inches.
Hope I helped. :)
Note: I'm still looking into this. There may be an earlier release of a plush toy, note that. I'm still checking. :3