I think the problem here is that 192bits is the maximum key size supported for 3DES in C# (and elsewhere I assume)
http://msdn.microsoft.com/en-us/library/system.security.cryptography.symmetricalgori...
Have you tryed setting the key to a 16 byte/char value?
The only other supported size seems to be 128bits. If you want somthing stronger you'll have to use a different algorithm, Aes/Rijndael.
If you look TDES up on Wikipedia, it seems the maximum key size is 168bits so I'm not sure how Microsoft have implemented it.
I suspect it was omitted from your example for brevity but you should probably hash your key from a good dose of salt. There are several RNG/HSH algorithms for this in the crypto namespace.