You know, I hadn't quite put two and two together in that way, but that assessment seems to have a lot of truth behind it. I can appreciate why they might avoid nailing down specific technical minutiae - the "black box" approach for futuretech is an awesome one when done well - but this almost feels more like an empty box to me.
Still, I would hope that there was some idea of the size involved... at least on the part of the modelers. But I can accept that there may be no specific, exact official size.
Well, for me this changed the dynamic of how I can watch this movie. It was a nice story and a fun ride, but seems to have been intended for only one or two viewings (or maybe more if considerable time is provided between them). This is how the average movie goer see most movies, and this movie was directly targeted for that audience. When viewing this movie in that mind set, I found it quite enjoyable (but I'm a major lover of movies in general, so I just watched this the same way I watch other non-Trek movies).
Sadly, some where in the process of figuring out how they were going to make this film, JJ got it in his head that only people who had no attachment or previous love of Trek would be able to work on this movie (which is why Eaves and Mandel were the only people with previous Trek experience). Even now, the people who had worked on this movie wear their dismissal of previous Trek as a
badge of honor when giving interviews (understandable, as it was their main qualification). Additionally (from what I can tell), JJ believed that previous Trek suffered under the weight of technical minutia and that to the best of his abilities this movie would be divorced from it for (as he believed) the sake of the story.
Personally, I don't think that setting a size for the Enterprise would have made any difference, but this movie is a work of art and to enjoy that work I think it is best not to question the artists choices too much while viewing it.