That is crucial, because a 5x5 cannot decode a sequence that was originally encoded with a 6x6 matrix.
We don't exactly need a 6-letter word; just any keyword that can be used to decode it correctly. If you look at the link I previously posted, note that the example is a very short keyword but a lengthy phrase. Think of the keyword as the key, and the phrase being the door. If we don't have the correct keyword, then we cannot "open" the door.
That being said, we should really experiment with different keywords. We may not even know what that keyword is until we play the map ourselves. I have a feeling it's not too simple, but not extremely difficult once we find it.
This Is incorrect, we do need a 6 letter word because that's how the letters are broken up into 6 letter horizontally, it does not matter how many vertically. now it is a ADFGX cipher because of the letters in the encryption there is no v and yes you can use a 5x5 Polybius square to solve it,
here is an example
_ A D F G X
A a b c d e
D f g h j/i k
F l m n o p
G q r s t u
X v w x y z
So looking at this we can see that the letter pair AA=a AD=b AF=c and so on.