I am looking for any extension for google-chrome which shows the size of loaded page. For example when you go to the www.foo.com it shows you the page which you fetched is 5 Mb . Is there any chrome extension to do that?
There's built-in functionality to do this, although the UI is rather obtrusive for normal browsing.
If you hit F12, you'll open the developer tools. Once they're open, click on the Network tab (red circle in screenshot), then browse to the page you want to size. You'll see a list of everything which loaded, along with the respective sizes (highlighted in the screenshot). At the bottom it shows the total amount loaded (also highlighted)
The size of the developer tools can be changed by dragging the border, or they can be split out into a separate window. For this screenshot I made them as large as necessary to show as many of the loaded files as possible, but it's possible to make it much smaller.