Q: How do I determine the storage size of the various sites in my SharePoint 2007 site collection?

Believe it or not, there’s no easy way to determine how much space is being used, with any degree of detail, from within SharePoint. So, most folks use some kind of third party tool, like this one:

http://www.thesug.org/blogs/lsuslinky/SSM/Pages/default.aspx

…or this one:

http://spusedspaceinfo.codeplex.com/

There are other options as well.

1. SharePoint Designer: Its reports show the size of objects within a site. However, you'll have to break out your calculator to tally it all up.
2. STSADM: The "enumsites" operation produces xml output that includes storage size for each site, including MySites.
3. Custom development: I like the solution this guy came up with: http://blog.sharepointdevelopment.nl/category/Web-Parts.aspx
   He developed a custom web part that lists all of the sites in the current site collection and the storage they use.

Actually, I like that last one so much that I made a finished solution for it, which you are welcome to take and use. (Disclaimer: I bear no responsibility for any negative consequences that arise from use of this software in your environment.) 

The solution file is here: http://sites.google.com/site/mctclassnotes/utility/SiteStorageWebPart.wsp (Install: "stsadm -o addsolution -filename SiteStorageWebPart.wsp", deploy: "stsadm -o deploysolution -name SiteStorageWebPart.wsp -url [YourSiteCollection] -immediate -allowGacDeployment" )

The source code, originally developed by Nick Boumans, is here: http://sites.google.com/site/mctclassnotes/utility/SP_Utilities.zip