In IMAP it depends on IMAP server supporting GETQUOTA command. Not all servers support it, you can check which extensions remote server supports by using Imap.SupportedExtensions method.
Quota quota = imap.GetQuota("");
long current = quota.Storage.Current;
Assert.AreEqual(15728640, quota.Storage.Maximum);
// you can also iterate over quota.Limits directly:
Assert.AreEqual("STORAGE", quota.Limits[0].Name);
long current = quota.Limits[0].Current;
Assert.AreEqual(15728640, quota.Limits[0].Maximum);
In POP3 you can use Pop3.GetAccountStat:
AccountStats stats = pop3.GetAccountStat();
Assert.AreEqual(2, stats.MessageCount);
Assert.Greater(stats.MailboxSize, 0);