What is the standard size for a microwave that would be attached over the stove and under the wall cabinets?

Question

How is the microwave affixed. I have one but I think it is too large. I want it up off the counter because it takes up a lot of space. Considering buying another one if I have to.

Answer 1

A standard over range microwave is 30" wide by 17" plus or minus a bit tall.  36" wide units are also available to go over a wider stove.

These are hung from the cabinet above with mounting brackets that come with the microwave.

The more important point is you can't simply take a standard countertop microwave and mount it over your stove.  The overrange units have a vent hood built into the bottom to take care of the steam, heat, and grease from the range below.  A standard microwave without this feature would not fair well.

Answer 2

if it is a microwave/hood combination, it should be at least the width of the stove.  

they generally come with a fairly simple mounting bracket.  This attaches the bottom.  the top is attached directly to the cabinets above.  the microwave should also come with a cutting template for where to place the holes in the cabinet.