There were basicly two studio models ILM built for the star destroyer.
#1 was about a 3.5' long model build in 1976 or so for SW:A New Hope...front face of the bridge section would be about 4.5-5" wide
#2 Was a larger 8' model built for SW:ESB...Bridge section approx around 12" wide.
Also for SW:ESB there was a large model of the back of the portside bridge. Used in a couple of shots in the film (once when the SSD is introduced, and later when we first see the 32" Millenium Falcon model attached to a Star Destroyer)...This was only the portside half of the back of the bridge section. Would have been about 7' wide, or 14' if the whole thing were contructed.
For SW:ROTJ a 4-5' wide model of the complete bridge section was constructed. Dressed as the Star Destroyer Avenger, then later redressed as the SSD Executor bridge.