Using this example on how to get the UNC path, you could just return the FreeSpace property, I've modified the code below:
ManagementPath path = new ManagementPath(@"\" + System.Environment.MachineName + @"\root\cimv2");
ObjectQuery query = new ObjectQuery("select * from Win32_LogicalDisk WHERE DriveType = 4");
ManagementScope scope = new ManagementScope(path, new ConnectionOptions());
ManagementObjectSearcher search = new ManagementObjectSearcher(scope, query);
foreach (ManagementObject o in search.Get())
{
Console.WriteLine(o.Properties["FreeSpace"].Value.ToString());
}