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

<script type="text/javascript">
  var page_port = null;

  chrome.extension.onConnect.addListener(function(port) {
    port.onMessage.addListener(function(data) {
      // Assign the content-script port to a global.
      // Dunno if this will do the trick, but now we know what
      // port to talk to.
      page_port = port;
    });
  });

  // Real men use jQuery
  document.getElementById("button").onclick = function() {     
    // This fires off the message to tell the content-script
    page_port.postMessage({width:800, height:600});
  }
</script>

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

resizer.js

// Connects up a message listener when the page loads
// This will receive the port.postMessage() from the extension
var port = chrome.extension.connect();

port.onMessage().addListener(function(data) {
  // Resize your shit here
});

// Fire off a message just to say "hi" to the toolstrip
port.postMessage({});