There are quite a few that are better than explorer, I really don't see how people continue to use it, its functions are cumbersome when working with lots of files.
Total commander is probably the most well known, click a folder and press space and it will display size or you can download one of the plugins that do it for you.
http://www.ghisler.com/
Directory Opus is another one, it has a feature that will display a bar graph for each folder to show size .
http://www.gpsoft.com.au/
Features overview of directory opus:
http://nudel.dopus.com/opus9/index.html#pagecontents
I like opus the best but it isn't cheap at $65. It does let you do just about anything with files and other things like ftp, network, etc.