I am just trying to build a UE4 4.26.0 Dev-Rendering Version from GitHub.
After downloading the files, extracting, building etc - the folder size becomes extremly large.
What can I delete after everything is compiled, without destroying the working 4.26.0 Dev-Rendering Version?
Probably, Build, Extras, Binaries, intermediate and saved. maybe stuff in here https://github.com/EpicGames/UnrealEngine/blob/release/.gitignore. Extras is subject to what your doing.
This is assuming, you are talking about a project created by the engine version and not the engine. I would leave the engine as is, and delete things from project if needed. Engine is large. Lots of stuff there, fairly normal.