Keeping a very large partition is not advisable. A partition of 20GB each is just about fair.
Just do this.
- Create a small text file with one character.
- Right click on it and see the properties.
- See the difference between size and size on disk.
If you keep a very large partition, this "size on disk" will be more. So even if the file is only 1 byte, it will occupy more space. (Read
here on why this happens.)
But then you can't keep making paritions of 2 GB to save on this. So strike a right balance and make reasonabe number of partitions of reasonable size. As I said, parition of 20GB each in a 80GB drive is fair enough.
As for the lost 6GB, its not due to partitioning. sakumar79 has given the explanation.
There was a law-suit filed in US against Western Digital (HD manufacturer) for misleading public about the drive size

Read about it at the following links:
Bits, Bytes and Lawsuit
Western Digital settles drive size lawsuit