Passphrase generation

[Internaut]

Hi,
I am wanting to learn how to generate a passphrase that is at least as 
strong as the IDEA algorithm.  I have looked several other places on the 
web for an answer to this, but they all had different things to say that 
didn't add up (no pun intended :).
The IDEA algorithm it seems is 2^128 =  3.402823669209e+38 = 16 bytes 
(charactors).
The charactor count seems kinda small (I am presuming the 16 charactors are 
truely random).
Indeed, 128(ASCII charactor set)^16 =  5.192296858535e+33.
Is my thinking right here?
Is it better to do this- 94(printable ASCII set)^20 =  2.901062411315e+39, 
yielding 20 charactors?

Also, if you come up with a phrase and put enough (perhaps 5 or 6) ASCII 
nonsense in there for it not to be in any crack dictionaries, how random is 
that?  Is it only as random as the extra charactors you put in?  How would 
you calculate that?

Also, how many charactors do you have to add of a set to add its 
permulations (i.e. Does gibber&sh add all ASCII symbols to the equation)?

Thanks, Internaut