Do you have Package Control installed ? Install it that way, it is easier to do so and you will always get the latest version from github. However if you still want to install it manually you have to put it into C:\Users\<yourusername>\AppData\Roaming\Sublime Text 3\Packages\User. %appdata% is a path alias for C:\Users\<yourusername>\AppData\Roaming\
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you can also add the line manually in the user settings
"font_size": 20,