To determine the length of side \( a \) using the given information that side \( b \) of A'Leila's donkey's pen is 12 meters, we first note that \( a = 12 \).
Since it's stated that \( a = 12 \), it implies that if side \( b \) is also 12 meters, then side \( a \) would similarly remain 12 meters, as they both share the same value in this context.
Thus, the answer is:
12 meters