Translation:My car's window is not big.
In my car seems much more appropriate then the option given "of my car". That would never be said.
"the window in my car is not big" has a different meaning.
this sentence is definitely using 'of' to show ownership/posession. The car owns the window. so you could say "the window of my car" or "my car's window"
I wrote: The window of my car is not that large. It counted me incorrect because of the 'that'. I guess that was my fault.
What does "de" stand for?