Hoods should always be about 6" wider than the range top (3" on each side) when possible (and especially when using commercial strength gas ranges). This has a pleasing visual effect and is functionally better as well. Height of the hood above the range is governed by manufacturer recommendations, but typically 34"-36" is what we use. We then proportion the height of the hood cabinet to be pleasing based upon the overall conditions (totally depends on the design/ceiling heights/room proportions). You should always get drawings from your cabinet supplier/designer to confirm all of these proportions before ordering.