If I have 30 gb of space how much should I sell ?

what if 15 gb/20gb/40gb ?

What if I've one 2 hard disk of 60 + 60
or 20 +60

Very typical question.

How much amount of space should be kept free for optimum performance of linux ?

We have found that, no matter what size plan it is, users use little space - very few sites go other the 50Mb mark, so you can 'oversell' your diskspace, which just about all the hosts out there do.

Regards
Matt