Find page size from within wordpress

In single php page, the size served from a php page can be determine by putting ob_start(); at the top of the page and ob_get_length(); at the bottom of the page.

However, wordpress (and virtually all CMS) use multiple php files, so is there any way to determine a the size (Kbs) of a page served by wordpress from within wordpress?

I'm thinking to put ob_start(); at the top of the index.php and ob_get_length(); at the footer.php, but not sure if it is correct, and how it works.

Could someone shed a light for me on this problem?

Thanks!