Find big files on the harddrive
size_with_biggest_unit should be streamlined. Move the unit names into a list:

unit_names = [ "", "KB", "MB", "TB" ]



and iterate downwards:

    for exponent in range (4, -1, -1):
        if size &gt; 1024 ** exponent:
            unit_name = unit_names[exponent]
            return str(round(size / 1024 ** exponent)) + unit_name
    raise ImpossibleError


Same (almost same) applies to size_in_bytes.


Hardcoding \ as a path delimiter seriously impairs the portability. Prefer os.path.sep. 
Instead of returning a list, consider turning it into an iterator.
Re Unused variable 'subfolders', a pythonic way to tell that the variable is truly unused is to call it _. I don't know if

for folder_name, _, filenames in os.walk(root_folder):


would pacify PyLint, but it would definitely make reviewer happier.