X is a random 160 bits number (20 bytes). It's size doesn't vary with the length of the public key (512-1024 bits).

While this structure has the SerializableAttribute attribute the private key, X, is marked as NonSerializesAttribute so it's content cannot be divulged using serialization.

Namespace: System.Security.Cryptography
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0