While dial-up users are becoming more and more like blue moons, it is still a valid goal to optimize the overall download size of your pages.
However, browsers and dynamic server-side technologies are making it harder to estimate the overall size of your page, since even brute-force methods like right-clicking images and selecting properties fails to yield size information.
A quick way to do this in Internet Explorer is to select File|Save As... and choose "Web Archive, single file (*.mht)", save it to disk, and view the properties through Windows Explorer.
Firefox has a similar option; select File|Save Page As... and choose "Web Page, Complete." However, in the FireFox case, you will also have to sum up the file sizes of the supporting files (images, etc.) which are placed in a subfolder called <file name>_files.
Another option is to use the
Fiddler, an HTTP debugger/logger developed by Eric Lawrence and Microsoft.