I'm a little confused. My microwave cabinet is 30" total (see pic below).
What size trim kit do I buy for a built in look- 27" or 30" if the inside is 26 3/4"?
Generally, you'd order the trim kit for a 30 inch cabinet. What microwave are you getting? You will need to make sure that your cut out dimensions will work for your microwave and trim kit (check the specs). For instance, the KA countertop microwave requires an opening cut-out height of 17 inches in order to use the trim kit, and your opening height is only 15 1/4. If your opening is too big for your trim kit, you can always add filler pieces to make it smaller, but if your opening is too small, you'd have to go with a different microwave/trim kit (I think). I have the same situtation with my base cabinet microwave opening and can't decide what to do there.
A common solution for larger MW's is to order a simple two drawer cabinet a couple of fillers. Remove the first drawer, build a platform and install the fillers appropriately. Then install the MW and trim kit. Don't forget to read your MW's specs for the proper location of the electric supply for the MW and that it cannot be run from the 2 required countertop small appliance circuits.