How does one show that there are no wff's of length 2,3 or 6, but that any other positive length is possible? It seems obvious I just don't know how to express it as a proof.
Let L(S) be a length of an expression S.
Each sentence symbol x is a well-formed formula of L(x)=1.
For example,
For a wff a, $\displaystyle L(( \neg a)) = L(a) + 3$.
For wff a,b, $\displaystyle L((a \wedge b)) = L(a) + L(b) +3$.
Now, we have wff's of length 1, 4, and 5. You can check the remaining cases inductively and make sure that no wff's of length 2,3 or 6 is possible.