I have a little microwave in my office - a cheap 900W Emerson from Target (I think it was about $65). It works great for my use, which is heating up leftovers. The dimensions are 19" wide by 14-1/2" deep (not including handle) by 11-5/8" tall. The handle sticks out just under 2".

Lots of microwaves have a push button to open the door instead of a handle. I don't think you'd have much trouble finding a MW to fit on a shelf 15" deep.

I have a ge spacemaker and it sits on a shelf less than 12" deep.

I have a purpose-built microwave shelf in my kitchen, purchased as part of a RTA cabinet suite. The cabinet depth is only 12", like the rest of my uppers, but there is an extra deep shelf affixed to the bottom, probably 16", which allows a deeper microwave unit (even a cheapo one like I use) to be installed with required air clearances. I don't find it "in my face" when using the area, and it is so nice to have landing space on the counter below when I need it, which I never had when the MW lived directly on the counter in the old kitchen.