Here I have mentioned the code which can be useful to get the Content DB, Site Collection Usage details.

 

SPFarm farm                 = SPFarm.Local;
SPWebService service        = farm.Services.GetValue<SPWebService>("");
 
string webappName       = "";
SPWeb web               = null;
SPUserCollection owners = null;
string OwnerString      = "";
int count               = 0;
 
foreach (SPWebApplication webapp in service.WebApplications)
{
    webappName                                  = webapp.Name.ToString();
    SPContentDatabaseCollection DBCollection    = webapp.ContentDatabases;
 
    foreach (SPContentDatabase contentDB in DBCollection)
    {
        //ContentDB details 
        row = null;
        row = tableDB.NewRow();
 
        row[col1] = webappName;
        row[col2] = contentDB.Name.ToString();
 
        
            //Get the ContentDB size from farm database server
            SqlConnection con   = new SqlConnection(contentDB.DatabaseConnectionString.ToString());
            SqlCommand com      = new SqlCommand("select Sum(size/128) from sys.database_files", con);
            try
            {
                con.Open();
                row[col3] = Convert.ToInt64(com.ExecuteScalar());
                con.Close();
            }
            catch (Exception ex)
            {
                WritetoEventViewer("Exception(ContentDB size calculation from farm DB) : " + ex.Message.ToString());
            }
            finally
            {
                con.Dispose();
            }
 
        row[col4] = Convert.ToInt64(contentDB.CurrentSiteCount);
        row[col5] = Environment.MachineName;
        tableDB.Rows.Add(row);
 
        //Site collection details
        foreach (SPSite site in contentDB.Sites)
        {
            try
            {
                rowSite = null;
                rowSite = tableSite.NewRow();
 
                rowSite[Server]         = Environment.MachineName;
                rowSite[WebApp]         = webappName;
                rowSite[ContentDBName]  = contentDB.Name.ToString();
                rowSite[URL]            = site.Url.ToString();
                rowSite[Size]           = site.Usage.Storage; // in bytes
                rowSite[Quota]          = site.Quota.StorageMaximumLevel; // in bytes
 
                    //Get the Users from site collection's owner group
                    OwnerString = "";
                    web         = site.RootWeb;
 
                    if (web.AssociatedOwnerGroup != null)
                        owners = web.AssociatedOwnerGroup.Users;
 
                    if ((owners != null) && (owners.Count > 0))
                    {
                        count = 0;
                        foreach (SPUser user in owners)
                        {
                            string loginname = user.LoginName.ToString();
                            
                            //To remove extra characters from claims
                            if(loginname.IndexOf("i:0#.w|") >= 0)
                                loginname = loginname.Replace("i:0#.w|", "");
 
                            if (count == 0)
                            {
                                OwnerString = loginname;
                                count++;
                            }
                            else
                            {
                                count++;
                                OwnerString = OwnerString + "; " + loginname;
                            }
                        }
                    }
 
                rowSite[Owner]          = OwnerString;
                rowSite[ReadLocked]     = site.ReadLocked;
                rowSite[ReadOnly]       = site.ReadOnly;
                rowSite[CurrentResourceUsage] = site.CurrentResourceUsage;
                rowSite[WriteLocked]    = site.WriteLocked;
                rowSite[UsageBandwidth] = site.Usage.Bandwidth;
                rowSite[UsageHits]      = site.Usage.Hits;
                rowSite[UsageVisits]    = site.Usage.Visits;
 
                tableSite.Rows.Add(rowSite);
            }
            catch (Exception ex)
            {
                WritetoEventViewer("Exception(Site Collection details collection) : " + ex.Message.ToString());
            }
            finally
            {
                owners = null;
 
                if (web != null)
                    web.Dispose();
 
                if (site != null)
                    site.Dispose();
            }
        }
    }
}