Why Reward Size 50 (21 Million Coins)?
The figure of 21 Million is completely arbitrary. It could have been any other number and it would make absolutely no difference, as long as the inflation schedule remains the same.

However, the inflation schedule is universally agreed to be one thing that can never be changed with Bitcoin. Any new code that extends the inflation will no longer be "Bitcoin" and will not be accepted by users of Bitcoin.

To make any change, it is not enough for the majority of nodes to modify, all nodes must eventually change. Usually it is enough for the economic majority to accept the change to trigger this, but with a change like this the worst that could happen is a fork to two different networks.