Moss 2007 get the site usage data using API , follow the simple steps to get the usage data.
string stURL = “http://mysite.com/”;
using (SPWeb spWeb = new SPSite(stURL).OpenWeb())
{
Response.Write(” Current site collection usage in MB = ” + (spWeb.Site.Usage.Storage * 1.0) /(1024 * 1024 * 1.0 ));
}