It seems to use a variable key size (it seems there isn't a "default" size) :
http://en.wikipedia.org/wiki/Blowfish_(cipher)
"Blowfish has a 64-bit block size and a variable key length from 32 bits up to 448 bits"
Here is an example using a 448 bits key : http://www.php.net/manual/en/function.mcrypt-encrypt.php#96635
Looking on other sites, MCRYPT_BLOWFISH_128, MCRYPT_BLOWFISH_192, MCRYPT_BLOWFISH_256 and MCRYPT_BLOWFISH_448 should work too (didn't try ^^)