How much of disk space is required to install RedHat linux..
And please say me which among linux versions below is best,
Is it RedHat or Fedora

No way to answer either question.

First, your installation size will vary, depending on what you install. Need X? KDE/Gnome? Optional packages, bare distro, etc.??? Disk space is cheap...if you plan on a 20GB minimum size, you should be safe.

And "best" is a relative thing. What are you looking to do? What environment? What hardware??? Fedora is updated more frequently, while RedHat is geared more towards servers. And if you're not going to PAY for RedHat, then don't use it. Load CentOS instead.

What are you trying to do - desktop or server? Are you married to Red Hat for some reason?

If you're looking for a desktop system, you probably want Fedora as it's more bleeding edge. I believe Fedora 13 is the latest stable version, but 14 might be out soon. The latest release notes have the hardware specs.

If you want a server (or your studying for your RedHat Certifications), you probably want CentOS, which is a clone of Red Hat Enterprise Linux (just missing the trademarked Red Hat name and logo). Here's a look at some of the system requirements.