bounded length CoNP proof
Assume that $A\in NP$, and let $V$ be a verifier for it. That is, $V$ takes as input $(x,y)$ and outputs 1 if $y$ witnesses that $x\in A$. That is, we can write $A=\{x:\exists y,\ V(x,y)=1\}$.

We show that $A\in coNP$, by showing that $\overline{A}\in NP$. We construct a verifier $V'$ for $\overline{A}$ as follows:

Given input $x$, $V'$ expects a witness of the form $(w_1,y_1,...,w_{n^3},y_{n^3})$, where $|x|=n$, such that $y_i$ is a witness that $w_i\in A$, and for every $i$ we have $w_i\neq x$. That is, a witness for $x$ not being in $A$ is simply a list of all the $n^3$ words that are in $A$. Clearly verifying this can be done in polynomial time using $V$ (which we assume to run in polynomial time).