What should be the minimum Space requirement for the new partition.
Hi, good question, but one which doesn't really have a right answer;
As there are many different kinds of Linux (all made by different people), they all require different amounts of space; for example, Puppy Linux is only 50MB, can be installed to a USB stick, and runs in RAM, which makes it really, really tiny and fast!
At the other end of the scale, a
distro (thats the term used to refer to Linux 'versions') that you are more likely to be using is
Ubuntu, which needs about a minimum of 4GB, but to be of any use, you're looking at at least 10-20GB, that does my Linux Mint nicely.
i will use linux mainly for ethical hacking purpose. Not more than that. also which version of Linux will be useful for me.
As always with Linux, there are specific distro's available for specific needs; a great one for hacking is called
BackTrack 3. That whole OS is designed for hacking, leagally of course

! And that requires about 1GB, though can be run off a USB stick so as not to effect your HD.
Although BackTrack 3 is designed for hacking, you might want to try a more easy-to-use distro like
Linux Mint, which is what I use, that is based on Ubuntu so needs about the same space as describe above.
there are a lot of versions. i am confused??
Yes, there are, and thats simply because Linux isn't an OS like Windows is, the only thing common between all Linux OS'es is the Linux Kernel, which is the like the core of it that makes everything work, all the rest is just added onto that, thats why its so easy to make so many versions of it.
Also, Linux isn't made by one person or company, each one is made by a different group of people, which is also a reason why there are so many kinds or 'flavours' of Linux.
Hope that helps you, if you need any other help, feel free to ask.