In SharePoint 2010 we can find size of SharePoint Site using STSADM Command as below:
1. cd "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:
2. STSADM.exe -o enumsites -url http:// <your site name>
It will display the size of your site under the tag "StorageUsedMB".