Neil from Naughty Dog said that the download will be 26 GB (give or take, most likely give). With that you will not need double the space to download, just 26 GB free space. He also said that the game itself will be released on their servers which would promote faster download speeds (yay!).
And yes at 50% the game will be playable; however, I do not know what it does to the quality or if the game will continue to download.