In MOSS2007 we can get to the STSADM by opening windows command prompt and changing directory to:

C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\BIN

In SharePoint 2010 we can find this at location:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\BIN

Now to get the size of your site type the following command on the prompt:

STSADM.exe  -o  enumsites  -url http:// <sitename>

It will display the size of your site under the tag “StorageUsedMB“.