Carbon dioxide is a gaseous molecule while $Si{O}_2$ is a three-dimensional –network in solid because

$C{O}_2$ molecules are held together by strong van der Waal’s forces where as $Si{O}_2$ is an ionic solid

$C{O}_2$ has a structure like that of $Ca{C}_2$ and $Si{O}_2$ has a rutile structure 

Carbon forms $\mathrm{\pi }$ bonds with oxygen and overlaps, $\mathrm{\pi }$ bonds between silicon and oxygen is not efficient

$C{O}_2$ molecules are held by 2p -2p  $\mathrm{\pi }$ bonding and $Si{O}_2$ forms an infinite three-dimensional structure.