Before creating game object
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var screenWidth = Math.max(window.innerWidth, document.documentElement.clientWidth);
var screenHeight = Math.max(window.innerHeight, document.documentElement.clientHeight);
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Then set the size to screenWidth/screenHeight at creating the game object

Then to get the game width/height use:
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game.world.width
game.world.height
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or
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screenWidth
screenHeight
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