take the measurement of few different model then see if their size is the same

usually the width is between 850mm to 900mm, while smaller hood is narrower, if you want to fit large household hood then leave a width of around 910mm

the cabinet on top you can leave the bottom board empty with just enough support for the carcass regidity, carcass depth of 350mm should be enough for most hood

actually are you designing your own cabinet? if not just let the contractor or designer do their job and tell them to do larger gap/cutout for you