what is the minimum free space required? and what happens with the server if the block chain size gets bigger than the free space we have in the server?
what "sever"? there is no server in a decentralized network. there are only peers which are individual computers. each computer can have a different storage types (HDD, SSD, M2,...) and different total and free space available to it. and so far the PCs have been way ahead in their storage than bitcoin blockchain size.