A general rule of thumb is 1 inch of space on either side and the top, but there is some variation. Some require less space. Some come with trim kits that mask them and hide any gaps. When you have narrowed your choices, download the owner's manuals and/or installation inatructions form the maker's web site and see what they say.

As for 24" being "too small," there is no rule of thumb nor are there kitchen police who will enforce a MW size. (Well, maybe on the house-hunter shows, where people say things like "I can't buy this house because ... " the microwave is too small, too old, too lacking in stainless steel fascia, too whatever.)

That I have a 23.5" GE Spacesaver II, may be fine for me for how I use a MW but somebody else may want/need a much larger one or may want/need a speed oven type device like an Advantium. So, figure out what you want to do with the MW and proceed accordingly.

Just an idea which may or may not appeal to you: I have the Monogram branded space saver, installed hanging from the cabinet above, no side clearance, and just a filler strip along the bottom. No shelf; we bought one but ended up not using it as the bottom is where it vents and we allowed no air space elsewhere. Gives it a somewhat built in look without any trim kit, unless you get down low and look up.

As for size, I was a little concerned about the ~1 sq ft but it has not been a problem for us. Large plates (like my 13" pasta plate) and the largest of my nesting glass mixing bowls fit fine. The only thing I've noticed not fitting so far was last weekend when my wife was trying to reheat an extra large Dunkin Donuts coffee. Too tall. We use our MW regularly but mainly just for simple reheats and individual frozen dinners. Perhaps some folks who use it more extensively (not sure for what) would find it too small, but we have no regrets.

Thanks for the responses. We are also minimal users of the MW. Otherwise, the thing would have been specified 6 months ago along with the other appliances. Now we have to get drawings to the cabinetmaker and I'm thinking he's going to want something more specific than "throw it on that shelf to the left of the fridge."

We did get it together enough to get the receptacle in roughly the right spot. I like the looks of yours, foodonastump, but I think we need ours to sit on the shelf because the mixer and food processor sit on the counter below and there's an undercounter light. Otherwise, I'd steal that idea in a second! (Am I correct that there's no place to hang a UCL in this installation?

So I did some very casual searches last night and it looks like allowing 2 ft width will not be very limiting. A few are slightly over, but it's not like I have my heart set on a particular model. It does need to hold a plate, a soup bowl, once in a while a mixing bowl. I've always had one on a counter with plenty of airspace to the sides and above, so for all I knew "everyone knows" that you need 2 inches, or some such thing. Guess not!

Actually I have a small strip along the back of the filler piece to hold an LED light strip. Could do something in the middle to hold pucks if that's what you prefer. Doesn't have to be 100% open under there I figure.

I do have to correct one thing - my pasta plate is more like 11.5 inches - I'm thinking 12 is about the max in that particular MW.