Different MW’s have different specs. The use of trim kits also changes things. Your specific MW will tell you.

@mackdolan thanks for that info. I’ve spent a little time online and have yet to see any microwave with clearance less than 3” on the sides. Yet a lot of inspo photos show the microwave very tight in the upper cabinet (no trim kit). Are there some models that need less clearance? Thanks

Pick a MW and buils according to the specs be aware that when it goes you will be limited to which one you replace it with

The micros in those inspiration photos are installed incorrectly. The heat/steam from a microwave needs someplace to vent, otherwise you will have damage to the microwave at minimum and quite likely damage to the surrounding cabinetry over time.

The optimal solution is to pick the opening dimensions that correspond to "worst case" (largest) for the microwave(s) you might consider. You can always put one in that requires less gap distances (although aesthetically it might be less desirable).

For example, if you have 2 microwaves under consideration and with clearances they need openings

24W x 16D x 15H and

25W x 14D x 18H

you should have an opening 25W x 16D x 18H. This would let you put either microwave in the space.

FWIW, I've been planning an IKEA kitchen for ages now, going back and forth about how and where to put a microwave in my small kitchen. I've been working with the idea that if I want a small countertop microwave in a cabinet, it should be 30 inches wide to maintain side-to-side clearance, and at least 15 inches high.

The depth clearance is a little more tricky, because a lot of microwaves seem to want 3-4 inches of clearance in the back, too. I might have to fudge that...

We have GEs largest microwave installed in an upper cab, almost four years ago. It gets used several times a day. Never had any issues. There’s just enough space on the sides to move it, if need be.

You‘ll have to tap on the pic to see the entire photo.