To find side $ a $ use formula:
$$ A = a \cdot b $$After substituting $A = 1048576\, \text{cm}$ and $b = 1024\, \text{cm}$ we have:
$$ 1048576\, \text{cm} = 1024\, \text{cm} \cdot a $$$$ a = \dfrac{ 1048576\, \text{cm}}{ 1024\, \text{cm} } $$$$ a \approx 325.9486 $$