To find side \( a \) using the equation \( a = \frac{1}{2} b \), where \( b \) is given as 12 meters, we can plug in the value of \( b \):
\[ a = \frac{1}{2} \times 12 \]
Calculating that gives:
\[ a = 6 \text{ meters} \]
So, the length of side \( a \) should be 6 meters.