There is not a single formula for this. It depends on the style, size, location, and height of the space.
Personally I find most hoods horrendous. Most of them look like spaceships that have landed in our kitchens. So my general rule is as invisible as possible.
All hoods need to be a min. of 700mm above the Hob/Oven. The size should match the size of the hob/Oven.
Martins Camisuli Architects
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