I can not see a comprehensive answer to the first question but will answer the next three
Why does fuel consumption decrease with increasing aircraft
Because of an improved ratio of true speed to total drag. See question 2
What is the relation between an airplane's altitude and the drag it
With reduced air density more speed is ...
The Helios was an electric powered prop driven aircraft that achieved an altitude of over 96,000 feet and maintained it in level flight for more than 40 minutes.
as altitude increases
You maintain wing lift with constant IAS until the wing airflow approaches the speed of sound (TAS increases). Shaft power output must increase to spin prop faster to ...