Most microwave and oven set ups require you to leave at least 30 inches between the cooking surface and the bottom of the microwave. This lets you use even very tall pots while still allowing for the steam to vent through the bottom of the oven. Look at your oven and microwave’s installation manual for more specific instructions.
Twin microwaves and ovens also require you to not place the microwave too high. The microwave becomes dangerous to place and remove hot food from it if you place it too high. The microwave oven should not be above your head. Most installation manuals are approximately 66 inches from the floor.
Some microwaves require some space in between the backsplash and the microwave. Others will allow you to tile the backsplash right up to the bottom of the microwave or even behind it. Look at your microwave’s installation manual for more specific requirements.
Some microwaves are only 30 inches wide but the space in between the cabinets is 36 inches wide or more. If this is the case, fill the gap between the microwave and the cabinets with a filler panel kit. These are skinny piece of wood that match with your cabinets.