How much space should I partition my hard drive for windows 10?

I have a macbook pro retain with a 250gb ssd. I plan to only use the windows partition for games(minecraft, heroes of the storm, League of Legends for now).
the windows 10 iso is 4gb
Heroes: 10gb
LoL:2gb
minecraft:
I plan on getting an external SSD later on to store more games. Will 20gb be enough?

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