I have my MW in my pantry. Things you have to think about: You have to get an electrician to install the outlet there. You also have to make sure there is a shelf that is deep enough front-to-back to hold a microwave, and at a good height. Mine is about waist height which I find perfect. You also need a counter nearby to be able to set down the hot items quickly if need be. I have an inexpensive Sharp countertop MW, I think about $110 or so. Works great.

We are not frequent users of a MW. If we were, I would probably not have put the MW in the pantry cause it is an extra step to open the pantry door, and the pantry is not centrally located, but is off to the side in the kitchen. For us, it’s great.

Regarding ventilation for the MW, I always have the pantry door open while it’s in use. The shelf it’s on is also wider than the MW so there is plenty of air space around it.

As long as there is adequate air space around the microwave, I wouldn’t think being in a pantry would be a problem. Check the specifications from the manufacturer.

A MW in a cabinet pantry is a fire hazard. A MW on a shelf or counter in a walk in pantry is fine.

Micowaves can be built in over an oven or elsewhere.

The real issue is how often it will be used. If the answer is often, then it's best to find a space in the kitchen where meals will be prepared. If it's used infrequently locating it out of the kitchen may be workable but it's hardly ideal.

We are also very infrequent users of a microwave. Ours will be a counter model in the walk in pantry. I researched this a lot on Houzz and many are happy with this choice.

On the counter.

Do not place one inside a cabinet without multiple safety precautions.

I put my microwave in the trash 30 years ago.

Assuming you're talking about a walk-in/step-in, or reach-in pantry and not talking about pantry cabinets...be sure you have adequate ventilation as -- for removal of heat and moisture. Pantries should be dark, cool, and dry for food storage. MWs introduce heat and moisture.

Just leaving the door open may or may not work, it depends on where the MW is located within the Pantry. If it's in front of the door, that may work fine. But, leave the door to the MW open for a while to let the heat and moisture dissipate...ditto the Pantry door (even longer).

I wouldn't store any foods above it that may be sensitive to heat or moisture (e.g., pasta in just cardboard boxes)...move those items to elsewhere in the Pantry.

You say "adequate air ventilation"...what are you planning?

Regarding sizing...as mentioned above, you need a shelf deep enough to hold the MW with several inches behind it for air circulation. Ditto the width...width of MW + several inches. Landing space was also mentioned above.