Manifest.json

{
  "name": "Sizerator",
  "version": "1.0",
  "description": "The first extension that I made.",
  "toolstrips": [
    "my_toolstrip.html"
  ],
 "permissions": [
    "tabs"
  ],
 "content_scripts": [
    {
	"matches": ["http://*/*"],
	"js": ["resizer.js"]
    }
]
}

my_toolstrip.html

<div class="toolstrip-button" id="button">
	<span>800 x 600</span>
</div>

resizer.js

document.getElementById("button").onclick = function() {
  var port chrome.self.connect();
  port.postMessage({width:800, height:600});
}

chrome.extension.onConnect().addListener(function(port) {
    port.onMessage.addListener( function( data ) {
        window.width = data.width;
    });
});