To check the Size or the Storage space of a Subsite you will have to open the site Collection in Windows Explorer.
Look for the Subsite(Folder), Right click on that and select properties. It will show you the exact space of the Subsite. There is no way to check that from the Site settings page or from Central Admin.
Let me know if anyone has found other way to check the Size for Sharepoint Subsite.
Zoeb Shaikh's SharePoint Blog 