Chrome Developer Tools - Browser Size?

They change the way to show this tool, you can find it pressing in device mode

It's a bug: https://bugs.chromium.org/p/chromium/issues/detail?id=582421

Google Chrome beta branch has this bug fixed already and the fix is expected to be added to the next stable release (version 50).

As a temporary workaround, you can use this bookmarklet:

javascript: (function() {
  var v = window,
    d = document;
  v.onresize = function() {
    var w = v.innerWidth ? v.innerWidth : d.documentElement.clientWidth,
      h = v.innerHeight ? v.innerHeight : d.documentElement.clientHeight,
      s = d.getElementById('WSzPlgIn'),
      ss;
    if (!s) {
      s = d.createElement('div');
      s.id = 'WSzPlgIn';
      d.body.appendChild(s);
      s.onclick = function() {
        s.parentNode.removeChild(s)
      };
      ss = s.style;
      ss.position = 'fixed';
      ss.top = 0;
      ss.right = 0;
      ss.backgroundColor = 'black';
      ss.opacity = '1';
      ss.color = 'white';
      ss.fontFamily = 'monospace';
      ss.fontSize = '10pt';
      ss.padding = '5px';
      ss.textAlign = 'right';
      ss.zIndex = '999999';
    }
    s.innerHTML = 'w ' + w + '<br />h ' + h;
  };
})();

I came across this trying to figure this out and couldn't find device mode as per Danamorah's answer. So I thought I would help save some time for a newbie and screen shot the icon to click so you can get measurements.

First, open dev tools, and then the top left corner you should see an icon shaped as a small tablet and phone. Click that and you can adjust measurements with the pixel count: