Choosing the appropriate hood size is essential for a satisfactory result.
OPTIMAL: The hood is larger than the cooking area
ADEQUATE: The hood is the same size as the cooking area
INADEQUATE: The hood is smaller than the cooking area

The height of the hood in respect with the cooking zone is an essential element to comply with what the legislation establish and guarantee the safety of the hood. That's why, according to the UNE-60335-2-31 standard:

1.  Wall hoods: max to 70 cm and min to 60 cm from the cooking zone.
2.  Inclined wall hoods: max to 70 cm and min to 60 cm counting from the center of the aspiration zone.
3.  Built-in hoods: max to 70 cm from the cooking zone.
4.  Island hoods: max to 70 cm and min to 60 cm.
5. Ceiling hoods: between 140 and 160 cm. 

Part of the suction efficiency and noise reduction depending on the project and length of the exit to the outside. That's why, we must:

1.  Avoid making a diameter reduction at the exit of the hood nozzle.
2.  Avoid making a diameter reduction near to the elbows, in straight sections and away from the hood.
3.  Avoid making rough diameter reductions on the tube.
4.  Avoid as much as possible the amount of elbows.
5.  Avoid an incorrect air circulation so that the smoke outlet be as much direct as possible.
6.  In case of putting elbows, try to make them as less acute as possible.

It's a basic requirement that the kitchen be provided with an air inlet to guarantee a good air circulation and, therefore, a correct hood functioning, since without this inlet and with the kitchen closed, a depression is generated thus reducing the suction efficiency of the hood.

In case of existing an air inlet for the gas installation, this would be useful and otherwise one must be created.