Bakagalas (バカガラス) aircraft schematic/size comparison
It is apparently inspired by the German Aircraft Me323D-1.
There are some schematics for a model of that here https://www.1999.co.jp/10548533

Google image search of  "バカガラス　模型"　brings up Japanese plastic models/drawings, if that helps.

The Japanese Wikipedia says there isn't any official documentation on the specific size of the plane, but a fan site (http://www.ghibli-freak.net/nausicaa/mecha.html) says it has an overall length of over 100 meters.

On a side note, someone made a real-life Nausicaa glider, which is awesome.
https://www.makery.info/en/2016/11/08/il-a-realise-le-planeur-de-nausicaa/