$\textbf{Answer}$

Bigger.

$\textbf{Analysis}$

Let's imagine that a same-sized circular metal is put inside the hole so we have a complete sheet. When this metal sheet is heated, the entire sheet will expand. So will the stuffed-in circular piece. Therefore, the original hole must become bigger in order to accommodate the expanded circular piece. 

$\textbf{Note}$

This brain teaser can be a physics exam problem. However, it can be solved using an easy-to-analyze model.