Unable to change RSA keysize?

From: Ayende Rahien (Ayende_at_nospam.com)
Date: 04/30/04 

Date: Fri, 30 Apr 2004 18:31:15 +0200

The following produce 1024 & 1024, why can't I change the key size?

RSA key = RSA.Create();
Console.WriteLine(key.KeySize);
key.KeySize = 4096;
Console.WriteLine(key.KeySize);