Dear SwissA330; sorry for the late reply. I am assuming (maybe wrongly?) that this project may be one of your early ventures into a model so large. If you are committed to this endeavor, I might suggest that you also purchase or borrow one of the commercially available, smaller scale, prebuilt examples of the aircraft to three dimensionally assist you in interpreting the measurements and drawings mentioned above by DC10 Levo.
Also, modern airliners typically have very smooth, finished surfaces. Balsa by nature is pretty easy to work with, especially in the larger 1:50 scale you are addressing, however, it is also very difficult to seal and finish-coat to an endurable "class-a" surface I think you would desire. I would suggest you inquire of local patternmaker and tooling supply shops in your area about the types of lightweight structural modeling foam (available in blocks of varying sizes, densities, etc) available for your project. The three dimensional surface development should progress a bit easier than using balsa, and after you are satisfied with your model shapes, a couple of applications of primer-surfacer and maybe a bit of spot/filler and putty will get you in the position to think about final paint, livery, etc. Structural modeling foam is quite stable and will not eventually telegraph surface deviations so common in balsa and even in properly cured mahogany. Furthermore, even thin parts such as wings and fins will not warp as wood will. Abscence of abuse, these items should remain relatively sound.
There is, of course, the option of using epoxy or polyester reinforced fiber, however, I personally feel that is quite a bit more labor intensive irrespective of whether you develop and cast a mold from a master model or simple coat a "core" model.
The professionals you talk to regarding these modeling supplies and finishes will
also supply you with health/safety guidelines...take their recommendations seriously, as even though working with such materials is not difficult, there are, indeed, (mostly) inhalation precautions that should be respected. Kind regards...Jack
all best; jack