Web game how to set screen size?

Hello! I am making a browser based game (like for Kongregate or Newgrounds) and I need to know how to set the screen size of the game. I want the game to be 640x480, how do I set this up? I have Unity 5 by the way.
Any help is greatly appreciated!

The resolution of a game built for webplayer is determined by the dimensions of the HTML element on the page in which it is embedded. i.e. your Kongregate/Newgrounds game template page, not at the point you export it from Unity.

You can set the resolution of the default HTML preview page by going to Edit → Project Settings → Player → Resolution and Presentation.