I am trying to find out how much minimum disk space is required for an OwnCloud installation? If I have a total space of 20GB on my server and install OwnCloud how much space will be available to me for data storage ?
ownCloud together with a normal Linux install will not need more then 1GB.
When you tested it out, please make a suggestion to add it in this list.
It's a very difficult question to answer, since ownCloud relies on other components that may or may not already exist on the system.
For example, before I even started using ownCloud, I was already running FreeBSD with Apache, MySQL and PHP (Basically a LAMP stack but with FreeBSD instead of Linux). That meant ownCloud itself took up about 100MB of space when I installed it. Pretty good!
But if you need to install a webserver, some kind of SQL database, PHP, and all the other dependencies, it suddenly becomes much larger. In fact, there are well over 200 dependencies for ownCloud on FreeBSD! Of course, I had most of these already installed, since the majority of dependencies are libraries that are also required by other applications, so it wasn't as bad as the dependency list suggests.
Depending on your OS and your current setup, the minimum disk space varies, so unfortunately it's impossible to give you an accurate answer. But 20GB should leave you with plenty of room, even if you don't have any dependencies already installed.
Hodyroff is correct, I'd be surprised if it took you more than a GB or two to install everything needed.