While there is an easy way to get the size of a Site Collection in SharePoint 2007:

STSADM.exe -o enumsites -url http:// YOURSITENAMEHERE > c:SiteList.txt

there seems to be no easy way to get that info for a specific site.

One thing you can do is to open your site in Windows Explorer (e.g. \YOURSITENAMEHERE) and navigate to the folder of the site you want to know the size of. Then simply right-click and choose “Properties” and that'll give you the size of everything within that folder plus any subsites that may exist. It's not a quick way to do things but it works.