How to choose grills for fireplace?

If fireplace manufacture didn't stated recommended area for intake and outlet, the grid should be chosen according to the fireplace output.

For ten-killowat output:

Air intake - the total of all incoming air ~400 cm2.
Air outlet - the total of all outgoing air ~600 cm2.

What could happen if wrong grilles selected?

Fireplace doesn't give all hot air.
Poor air circulation.
Internal structures may overheat.
Fireplace capsule may overheat cousing internal structures deformation.