but looking here:
https://www.blockchain.com/charts/blocks-size I see that block chain size is getting close to 300 GB
~
and what happens with the server if the block chain size gets bigger than the free space we have in the server?
As pooya87 said, there is no "server".
Second, if you look at the numbers, it's 70 GB a year and it can't grow with much more than that a year. The basic 500GB SSD you can get for under 100$ will be able to run the blockchain for another 3 years. By that time 1TB SSDs will be cheap enough for almost anyone to afford, and they will give you an extra 7 years of storage capacity.