1) In addition to the space needed by WinCC flexible, Windows also requires space on the hard disk;
e.g. for the swap file. The following formula has proven itself in the past: The size of the swap file = 3 x the size of available RAM.
For further information, refer to your Windows documentation

2) When installing one language. An additional 200 MB are required for each further language. In the case of different partitions for system and configuration: System partition approx. 700 MB, project partition approx. 1.3 GB.

3) When installing one language. An additional 80 MB are required for each further language. In the case of different partitions for system and configuration: System partition approx. 600 MB, project partition approx. 600 MB.