Good question. I'm not sure. Mcrypt handle the key length automaticaly... maybe Terra was inspired from this? 

Openssl, on the other side, has fixed key-length ciphers ie AES-128, AES-192, AES-256

Rijndael is hybrid. Automaticaly key length settings and optional fixed via setKeyLength().
Twofish only determines automaticaly the key length... i was not thinking about setKeyLength() when setting up Twofish :-) 
3DES the same. No setKeyLength() or a second optinal keyLength parameter in setKey().

Personaly i tend more to an optional public setKeyLength() in RC2, even i like in general the idea of the 2nd parameter in setKey() also. But will leave it at least by you.