Being open source allows the client to be peer reviewed. It also allows you to download the source code and compile it yourself in case you're worried that the source code is different from the distributed version of the application.
As of December 2015, the entire blockchain is over 50GB. The growth rate has been more or less linear for the last year when it grew in size by about 25GB. Thus it's reasonable to expect the blockchain to grow to 75GB by the end of 2016 and so on.