Some of it is based upon the actual measurements of the orignial S.S miniature.Reference books are super important,and over the last 10 years or so, some good reference materials have become available.Allot of that is also due to "the Internet". The Star Wars Chronicles, The Art of Star Wars series,Sculpting a Galaxy,and From Star Wars to Indiana Jones are all books that most everyone in here has,along with any other photo references you can find. Sometimes I think that the scales are based on the kit parts used on the kits,and if you are a HUGE fan of Star Wars(or any Sci-Fi) that you just imerse yoursef in the subject of interest. You do allot of H.W and find out all that you can on what you are wanting to make. There are also allot of Star Trek journels,magazines,technical books and web info that give out rough estimations of the scales the ships were made in.Cinefex Magazine(the old ones) have lots of good info and photos.I have a series of them myself that I often use for reference to detailing and weathering.