How much space is enough for Linux?

The base install of Linux requires about 4 GB of space. In reality, you should allocate at least 20 GB of space for the Linux installation. There is not a specified percentage, per se; it is really up to the end user as to how much to rob from their Windows partition for the Linux install.

Is 20 GB enough for Linux?

For just messing around and having a basic system, 20 is more than enough. If you download you will need more. You can install a kernel module to use ntfs so that space can become available to linux as well.