Almost every negative that I like and end up printing, I print, and them mount on an 11x14 board. I have 11x14 archival storage boxes, and the walls in my house and office have several 11x14 frames. I rotate prints through the frames. This means that most of these prints end up being on 8x10 paper, sometimes 11x14 paper, and cut down to a smaller size. Most of my negatives are 6x6 or 4x5 and I do not have any compulsion to print at the same aspect ratio as my negative - I almost always crop.
Some negatives I print larger - and in those cases, it depends on the image. Some work better at 11x14. some on 16x20 paper, and sometimes 20x24. On those images, the exact image size, and the size of the mat that it is mounted on are determined by what I feel works best with that particular image.