(Sorry for my poor english skill..)

Let $N$ be a large integer and the set $X$ be the subset of $\mathbb{Z}/N\mathbb{Z}$. For two sets $A$ and $B$, we define \begin{equation} A+B:=\{a+b : a\in A, b\in B\}. \end{equation} Is there a bound of size X that satisfies $X+X=\mathbb{Z}/N\mathbb{Z}$?