I really don’t think there is a definitive answer to this question. The only description we had at the time was that it was “souped-up” and bigger than the Enterprise. The size the miniature was built to, was determined by the camera guys as being the optimal size to shoot. As I recall NIlo did that size comparison chart, but it wasn’t necessarily followed. The issue of true scale wasn’t a consideration. Budget, ease of use, art direction and dramatic intent were what influenced the construction of the ship. The current techniques of the ILM model shop also swayed the look of the ship. If I recall correctly (it was a LONG time ago) some of the detail in the landing bay and the grid frisket patterns used on the hull were left over under-construction Death Star etched brass. Nilo Rodis and Dave Carson had done a number of designs for the ship that I had built quickie prototypes of. I did finish a bit early and Nilo told me I could take a crack at one myself. When they were presented to Nimoy he chose the one I did because I think it looked the most like the Enterprise. Although the prototype had the basic look of the final ship it was a bit frumpy. The model shop supervisor, Steve Gawley, passed the prototype over to Mike Cochrane (I think) who under Steve’s guidance did a beautiful set of blueprints. He really improved and polished the design. The model was built directly from those drawings.
Someone mentioned the size of the spacedock doors and how the Excelsior could never fit through them. It’s funny how we never even considered that! The Enterprise was scaled to the doors to make it a dramatic exit. That space dock interior was really a piece of crap. I came up with the idea of doing artwork and large size prints to use as wallpaper for the interior detailing and that stuff was always peeling off. Another funny memory of that set was that it was like a giant “whisper dish” like you would experience at a science museum. If you were on one side talking your voice would be super loud on the other. We farted a lot on that set to much hilarity!
Many years later the Excelsior came back to ILM to be re-fitted as the Enterprise B. The upper part of the primary hull and the lower part of the secondary hull were completely replaced. Those piece of the ship were changed enough that replacement was easier that retrofitting. Also, the fiberglass of both of those sections was starting to buckle. In case you are wondering, the original hull pieces do still exist. ☺ I’m sorry I couldn’t be of more help. From the comments I have read here you all know a lot more about this issue than I do. Cheers.